The central effects of morphine on fetal breathing movements in the fetal sheep

The fetal respiratory and electrocortical effects of 0.6 microgram to 600 micrograms of morphine, administered into the lateral cerebral ventricle, have been studied in chronically catheterised, unanaesthetized fetal sheep at 115-135 days gestation. Morphine at 0.6 microgram had no effect on breathing movements or electrocorticographic activity, and at 6 micrograms induced a period of apnoea (43-122 min) but had no effect on electrocortical activity. Intravenous naloxone (2 mg bolus and infusion of 2 mg/kg/h for 2 h) to the fetus had no effect on this apnoea. Morphine at 60 micrograms induced an initial period of apnoea (30-65 min) followed by episodic but significantly deep breathing movements with no effect on electrocortical activity and at 600 micrograms induced an initial period of apnoea (22-95 min) which was followed by deep, irregular and continuous (126-302 min) breathing movements. During the apnoea electrocortical activity initially remained cyclic, but as apnoea progressed there was a gradual reduction in the voltage of the electrocorticogram to a low voltage state. Intravenous naloxone (2 mg bolus and infusion of 2 mg/kg/h for 2 h) reversed both the respiratory and electrocortical effects. The hyperventilation was also inhibited by hypoxia. Naloxone alone had no effect on fetal breathing activity.     

Almitrine inhibits breathing movements in fetal sheep in utero

Whilst hypoxia stimulates fetal peripheral chemoreceptors, fetal breathing movements do not increase as hypoxia also has central effects. We wondered whether specific stimulation of the arterial chemoreceptors by almitrine would produce a stimulation of fetal breathing movements. When almitrine was given to 5 intact and 3 peripherally-chemodenervated fetal sheep in utero, fetal breathing movements rapidly ceased for 1-12 h. There was also a decrease in the amount of time spent in low voltage electrocortical activity. The effects of almitrine are therefore similar to those of hypoxia, and are independent of the peripheral chemoreceptors. Thus it may be a valuable tool in the study of the control of fetal breathing.     

Effects of maternal indomethacin administration on fetal breathing movements in sheep

To determine the role of prostaglandins in the control of fetal breathing movements, we infused indomethacin (5 mg/ml; 25 mg/kg per day) into the maternal femoral vein for 70 h in 5 pregnant ewes. There was a significant increase in the incidence and amplitude of fetal breathing movements beginning within 2 h reaching a peak at 8-10 h. It then diminished and was no longer present by 20-70 h despite continued indomethacin infusion. Maternal glucose concentrations were increased at 8 and 16 h following the initiation of indomethacin infusion. The data suggested that the previously reported effects of cyclo-oxygenase inhibitor on fetal breathing movements are transient and do not continue beyond 20 h.     

Effect of hypoxia on the excitability of two cranial reflexes in unanaesthetized fetal sheep

In fetal sheep acute hypoxia causes a decreased incidence of breathing movements and motor activity, and the excitability of polysynaptic reflexes in the hindlimbs is depressed. To determine whether this inhibitory effect extends to other areas in the fetal CNS, we have studied the effect of hypoxia on two reflexes with cranial pathways. The digastric (jaw opening) reflex was elicited by stimulation of the dental nerve through a pair of stainless steel electrodes implanted into the mandible (4 fetuses). The thyroarytenoid muscle of the larynx was reflexly activated by stimulation of the superior laryngeal nerve by a cuff electrode (4 fetuses). Low level stimulation at 1.5-2 X threshold was repeated at approximately 2 min intervals for 3-4 h; the stimulation did not alter the pattern of electrocortical activity, breathing movements, or cause arousal. The amplitude of the digastric reflex was greatest during low voltage electrocortical activity; conversely, the amplitude of the thyroarytenoid reflex was greatest during high voltage electrocortical activity. Isocapnic hypoxia lasting 30-60 min (16 trials), in which the PaO2 was reduced to 12-14 mmHg, did not reduce the amplitude of either reflex. The reduction of thyroarytenoid reflex amplitude which normally occurred during low voltage electrocortical activity was not present during hypoxia. These experiments show that the inhibitory effects of hypoxia on spinal reflexes, breathing movements and motor activity do not include these cranial pathways.     

Stimulation of breathing movements in fetal sheep by inhibitors of prostaglandin synthesis

We studied the effects of inhibitors of prostaglandin synthesis on fetal breathing movements on 17 occasions in 11 lambs (gestational age 125-141 days). We gave 12 h infusions of sodium mechlofenamate (8.6-22.2 mg.kg-1) in 13 studies and indomethacin (21.8-38.8 mg.kg-1) in four studies. Results were similar with both agents and did not correlate with drug dosage. There were no changes in fetal arterial blood pressure, pH or blood gas tensions. We assessed fetal breathing movements by measurements of tracheal pressure for a control period of 224 h prior to and 208 h during the infusion of inhibitors of prostaglandin synthesis; their administration caused a marked stimulation of fetal breathing movements judged from the following four variables: (1) incidence of fetal breathing movements increased from 38.4 to 69.2% of the time (P < 0.001); (2) average amplitude of change in tracheal pressure during fetal breathing movements increased from 4.1 to 6.0 torr (P < 0.01); (3) maximal amplitude of change in tracheal pressure during fetal breathing movements increased from 8.8 to 13.4 torr (P < 0.01); and (4) the duration of the longest continuous episode of fetal breathing movements increased from 37 to 229 min (P < 0.05). Two fetuses had electrocorticogram (ECoG) recordings. In control periods, fetal breathing movements occurred only during low voltage, high frequency ECoG activity; however, during infusions of inhibitors of prostaglandin synthesis, fetal breathing movements occurred also during high voltage, low frequency ECoG activity. We conclude that inhibitors of prostaglandin synthesis stimulate fetal breathing movement in fetal sheep. These results suggest that a component of the prostaglandin system is a factor which inhibits breathing movements during fetal life.     

Apneic effects of cholecystokinin in unanaesthetized fetal sheep

The effects on breathing movements and sleep state of cholecystokinin octapeptide (CCK-8) and its antagonist, proglumide, have been studied in unanaesthetised fetal lambs of 124-142 days gestation. CCK-8 when given into a lateral cerebral ventricle as bolus injections of 10-500 ng caused dose-related periods of apnea ranging from 63-214 min. When given as a 100 ng bolus followed by a 50 ng/h infusion for 2 h there was a prolonged period of apnea lasting 331 +/- 56 min. There was no effect of CCK-8 when given in higher doses (1-50 micrograms). The antagonist proglumide reversed the apnea induced by CCK-8 infusion, but had no effect when given alone, nor did it affect the normal fetal depressive response to hypoxia. Neither CCK-8 nor proglumide had any effect on electrocortical activity. We conclude that CCK has no role in the inhibitory mechanisms causing the apnea associated with high voltage electrocortical activity or hypoxia in the fetus. Furthermore CCK does not appear to be involved in the regulation of sleep state in the fetal lamb.     

Effects of increased core temperature on breathing movements and electrocortical activity in fetal sheep

Core temperature of fetal sheep was raised by perfusing warm water through a loop implanted into the abdomen, or into the stomach via the oesophagus. Raising fetal temperature by 0.8-2 degrees C was associated with an increase in amplitude and incidence of breathing movements, and an increase in the proportion of breathing movements that occurred during high voltage electrocortical (ECoG) activity. Fetal hyperthermia was maintained for 8 h, but the augmentation of breathing movements did not last for more than 2-3 h. The results indicate that changes of maternal temperature caused by hot weather, exercise, fever, and possibly diurnal changes of body temperature could alter the amplitude and pattern of fetal breathing movements.     

The effects of raising intracranial pressure on breathing movements, eye movements and electrocortical activity in fetal sheep

Extra-dural or cerebroventricular intracranial pressure was measured in 7 unanaesthetized fetal sheep (123-137 days gestation). Basal intracranial pressure was 6.7 +/- 1.7 mmHg, but there were many transient increases of pressure in association with spontaneous changes of amniotic pressure, fetal intrathoracic pressure, and particularly when the fetal nuchal muscles were active. These spontaneous increases of intracranial pressure were often associated with cessation of breathing movements and change of the electrocorticogram from low to high voltage activity. To test whether increased intracranial pressure influenced breathing movements and electrocortical activity, intracranial pressure was raised either by occluding the superior vena cava for 1 min with an implanted extravascular cuff, or by extra-dural injection of 0.3-1.0 ml of 0.9% NaCl. Increasing the intracranial pressure 5-15 mmHg by either method during low voltage electrocortical activity caused cessation of breathing movements, electro-ocular activity, and change of the electrocorticogram from low to high voltage in a significant proportion of trials. We propose that natural fluctuations of intracranial pressure caused by compression of the fetal body or skull, by body movements or by uterine activity, may cause changes in electrocortical activity and breathing movements.     

Effects of hypoxaemia and hypercapnia on breathing movements and sleep state in sinoaortic-denervated fetal sheep

The role of the systemic arterial chemoreceptors in regulating breathing movements was determined in 7 chronically catheterized fetal sheep with carotid denervation and vagal section. Fetal hypoxaemia (delta PaO2 = -11.4 +/- 0.6 mmHg) decreased significantly the incidence of rapid-eye-movements (control = 26 +/- 1.5 min/h; hypoxia = 12 +/- 2.6 min/h, P less than 0.001) and breathing activity (control = 18 +/- 1.0 min/h; hypoxia = 8 +/- 1.1 min/h, P less than 0.001). However, the lag in onset of inhibition (approximately 8 min) was significantly greater (P less than 0.05) than for normal fetuses. The incidence of low voltage electrocortical activity was not affected. Hypercapnia (delta PaCO2 = 9.5 +/- 1.1 mmHg) increased significantly the incidence of rapid-eye-movements and breathing activity. Hypercapnia also increased the mean amplitude of breathing activity and reduced the average breath interval. Rapid-eye-movements and breathing activity were depressed significantly by hypoxaemic hypercapnia. These observations suggest that hypoxic inhibition does not require afferent activity from the aortic or carotid bodies nor from other chemoreflexes mediated by the vagus. However, such peripheral input may be responsible for a more rapid onset of inhibition in normal fetuses.     

Effects of hyperthermia on fetal breathing movements

High environmental temperature is known to impair fetal growth and development. We now report long lasting changes in fetal breathing activity following the exposure of pregnant ewes to an ambient temperature of 43 degrees C for 8 h. In 16 trials in 10 ewes (119-138 days gestation) heat exposure increased maternal and fetal core temperatures 1.5-2.0 degrees C, and the hyperventilation by the ewe produced a fall in fetal PaCO2 from 53.5 +/- 1.3 to 34.8 +/- 5.3 mmHg (P less than 0.05). Fetal breathing movements decreased in incidence during the hyperthermia but remained episodic (present during low-voltage electrocortical activity) with occasional brief episodes of breathing at high rates (greater than 4 breaths/s). However, 1-2 h after the end of heating, when maternal and fetal core temperature and PaCO2 had returned to normal, fetal breathing movements became continuous, and were augmented 30-100% in amplitude. Fetal breathing movements occurred during both low- and high-voltage electrocortical activity. The results show that a heat load similar to that experienced by sheep in sub-tropical regions in the summer months cause prolonged changes in the central regulation of fetal breathing.     

Effect of cigarette smoking on fetal breathing movements in normal pregnancies.

In 18 women with uncomplicated pregnancies smoking two cigarettes significantly reduced the proportion of the time that fetal breathing movements were present.     

The development of the somatosensory evoked potential in the unanaesthetized fetal sheep

Somatosensory evoked potentials were recorded in utero from 13 chronically instrumented fetal lambs (97 to 148 days of gestation) following electrical stimulation of the upper lip or upper limb. Several clear and reproducible peaks were observed. Following upper lip stimulation, peaks were seen with mean peak latencies of 9, 13.2, 17.8, 21.3, 33.8 and 206 ms at a gestational age of 125 days. Similar peaks, but of slightly later mean latencies, were seen following limb stimulation. These peaks demonstrated significant gestational age related falls in peak latencies (P less than 0.05). Several of the mid to late latency peaks, notably those occurring at 21.3, 33.8 and 206 ms, demonstrated changes (P less than 0.05) in both latency (longer in low voltage) and amplitude (reduced in low voltage) dependent on electrocorticographic state. Rate of stimulus presentation also had a significant effect on both amplitude and latency of several peaks (P less than 0.05) with this effect lessening with advancing gestational age. Evoked potentials can thus be successfully obtained from chronically instrumented fetal lambs and provide a useful indice for studies of neural maturation.     

Effects of moderate hypoxia on fetal electrocortical activity, eye movements, and breathing activity in sheep

Isocapnic hypoxaemia (delta PaO2 = -8.0 +/- 0.5 mmHg; delta CaO2 = -2.86 +/- 0.20 ml/dl) was produced in fetal sheep by having the ewe breathe for one hour a gas mixture (v/v) of 10.5% O2 and 1.5% CO2 in N2. Mean fetal heart rate, blood pressure, and incidence of low voltage electrocortical activity were not affected. However, the incidence of rapid-eye-movements and breathing activity was reduced by about 40%. Breathing movements during hypoxaemia had a mean inspiratory time, breath interval, and tracheal pressure amplitude which did not differ significantly from those during control experiments in which the ewe breathed air from the plastic bag. These observations suggest that hypoxia decreases the incidence of breathing movements but does not affect the amplitude or pattern of breathing activity and that it may reduce the incidence of eye movements and breathing activity through a common mechanism.     

Prostaglandin E2 inhibition of fetal breathing movements is not sustained during prolonged reduced uterine blood flow in sheep

Fetal breathing movements (FBM) are inhibited by both exogenous prostaglandin E2 (PGE2) and ethanol in sheep. Maternal ethanol exposure in late-gestation sheep also increases fetal [PGE2]. However, during prolonged reduced uterine blood flow (RUBF) when [PGE2] in fetal plasma is already elevated, FBM are not inhibited by ethanol. These experiments were designed, therefore, to test the hypothesis that the FBM response to PGE2 is also diminished during RUBF. PGE2 (594+/-19 ng.min(-1).kg(-1) fetal body weight) was infused for 6 h into the jugular vein of RUBF (PO2 = 14+/-1 mmHg (1 mmHg = 133.3 Pa); n = 7) and control (PO2 = 22+/-1 mmHg (p < 0.01); n = 7) ovine fetuses, and the effect on FBM, electrocortical (ECoG), and electroocular activities was determined. The infusion of PGE2 increased plasma [PGE2] from 881+/-162 to 1189+/-114 pg.mL(-1) in RUBF fetuses and from 334+/-72 to 616+/-118 pg.mL(-1) (p < 0.05) in control fetuses. FBM were initially inhibited by PGE2 from 22.5+/-9.4 and 17.9+/-6.5% of the time to 6.9+/-2.4 and 0.5+/-0.4% (p < 0.01) in RUBF and control fetuses, respectively. FBM remained inhibited in control fetuses throughout the infusion but returned to baseline incidence in RUBF fetuses in the last 2 h of the infusion. These results are consistent with the hypothesis that one component of the adaptative mechanisms of the fetus to prolonged RUBF is an altered response of FBM to exogenous PGE2. We speculate that the lack of a sustained inhibition in FBM during RUBF with infusion of PGE2 may be a result of an alteration in brainstem receptor function or number or local PGE2 removal.     

Effect of acutely-induced lactic acidemia on fetal breathing movements, heart rate, blood pressure, ACTH and cortisol in sheep.

Experiments were conducted in 8 chronically-catheterized fetal sheep at 125-135 days gestation in order to determine the effect of exogenously administered lactic acid to the fetus on fetal heart rate, blood pressure, breathing movements (FBM), electrocortical activity (ECOG), plasma immunoreactive (IR-ACTH) and cortisol concentrations. When fetal arterial pH decreased from 7.37 +/- 0.01 during the control period to 7.20 +/- 0.01, there was an initial bradycardia followed by tachycardia but no change in blood pressure. The amplitude of FBM increased 2-fold initially in association with an increase in PCO2 from 47.9 +/- 2.1 mmHg to 58.8 +/- 3.6 mmHg at 5 min into the lactate infusion. There was no change in the incidence of FBM or low-voltage ECOG and there was no change in the plasma concentrations of IR-ACTH and cortisol with the infusion of lactate. We conclude that the major effects of acutely elevating circulatory lactate concentrations in fetal sheep are to increase the amplitude of FBM and to cause an initial bradycardia followed by a tachycardia.     

Fetal breathing is not initiated after cord occlusion in the unanaesthetized fetal lamb in utero.

We investigated the role of cord occlusion in the initiation of breathing at birth using an extracorporeal membrane oxygenator system to control fetal blood gases independently of the placenta in 12 chronically instrumented fetal lambs. In group IA (n = 9; exp = 12) PaCO2 was kept constant (5.62 +/- 0.21 to 5.70 +/- 0.23 kPa) during cord occlusion. Group IB (n = 7; exp = 8) were cord occlusion experiments from group IA in which no fetal breathing movements had occurred; CO2 flow to the membrane was increased and fetal PaCO2 rose significantly (5.45 +/- 0.24 to 8.27 +/- 0.56 kPa). In group II (n = 7; exp = 12) PaCO2 was allowed to increase from 5.98 +/- 0.24 kPa to 8.09 +/- 0.48 kPa after cord occlusion. Within 5 min of cord occlusion, FBM did not occur in 11 out of 12 experiments in group IA or in 11 out of 12 experiments in group II. In contrast in group IB breathing did occur in 5 out of 8 experiments. When they occurred, fetal breathing movements were always associated with low voltage electrocortical activity. Our results do not support the hypothesis that the initiation of breathing within 5 minutes of birth is dependent on an inhibitory factor of placental origin. Furthermore these data suggest an association between the presence of breathing and a substantial rise in PaCO2.     

The effects of indomethacin and prostaglandin E2 on the ethanol-induced suppression of ovine fetal breathing movements.

A study was performed to examine the role of prostaglandins (PGs) in the mechanism of the ethanol-induced suppression of FBM, in which the objective was to test the hypothesis that fetal administration of PGE2 can suppress the incidence of FBM following reversal of ethanol-induced suppression of FBM by indomethacin, a fatty acid cyclooxygenase inhibitor. Instrumented near-term pregnant ewes received 1-h maternal infusion of ethanol (1 g/kg maternal body weight) followed 0.5 h later by a 3-h fetal infusion of indomethacin (1 mg/kg fetal body weight/h), and then a 2-h fetal infusion of PGE2 (400 ng/kg fetal body weight/min). Prior to drug administration, FBM occurred approximately 36.1 +/- 2.6% of the time. FBM were suppressed during the period of ethanol infusion (9.6 +/- 1.7%); the ethanol-induced suppression of FBM was reversed by fetal indomethacin treatment (77.5 +/- 14.1%); shortly after the onset of fetal PGE2 infusion, the incidence of FBM decreased to a 2-h mean incidence of 14.1 +/- 4.2%, which was similar in magnitude to that observed after maternal ethanol infusion. After the completion of PGE2 infusion, the incidence of FBM rapidly increased to a peak incidence of 83.4 +/- 19.2%, which was indicative of a prolonged effect of indomethacin on FBM. The data indicate that PGs mediate the ethanol-induced suppression of ovine FBM and that the action of indomethacin to antagonize ethanol-induced suppression of FBM is primarily due to its inhibition of PG synthesis.     

Electrical stimulation of the posteromedial thalamus modulates breathing in unanesthetized fetal sheep.

Having previously shown that lesions in the posteromedial group of thalamic nuclei abolish hypoxic inhibition of fetal breathing, we devised this study to identify thalamic loci that depress breathing by focal stimulation of specific sectors of the caudal thalamus and adjacent structures. Multipolar electrode arrays consisting of a series of eight stimulation contacts at 1.25-mm intervals were implanted vertically through guide cannulae into the caudal diencephalon of 12 chronically catheterized fetal sheep (>0.8 term), and central neural tissue was stimulated between adjacent contacts. Each site was stimulated repeatedly with increasing current searching for spatial and stimulus strength parameters for a reliable alteration in respiratory rate. Respiratory period increased when stimulation involved areas of the parafascicular nuclear complex (Pf), which more than doubled the mean period compared with the baseline of 0.90 +/- 0.19 s. The change in respiratory period was due to an increase in expiratory time, whereas inspiratory time and breath amplitude were not significantly affected. Breathing period and expiratory time were also increased when the stimulations involved the intralaminar wing surrounding the mediodorsal nucleus, the rostral central gray, zona incerta, and ventral tegmental area. Reductions in respiratory frequency occurred less consistently, with stimulation involving surrounding zones including the sub-Pf, ventromedial nucleus, and ventrobasal nuclear complex. These findings support the hypothesis that a restricted area of the posteromedial thalamus (principally Pf) constitutes part of a neuronal circuitry that modulates respiratory motoneurons.