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.     

Effect of morphine on breathing and behavior in fetal sheep

To define the dose response of apnea and breathing to morphine we studied 12 fetuses at 116-141 days of gestation using our window technique. We instrumented the fetus to record electrocortical activity (ECoG), eye movements (EOG), diaphragmatic activity (integral of EMGdi), heart rate, carotid blood pressure, and amniotic pressure. Saline and morphine in doses of 0.03, 0.1, 0.5, 1, and 3 mg/kg were injected in random order in the jugular vein of the fetus during low-voltage ECoG. Fetuses were videotaped for evaluation of fetal behavior. We found 1) that saline did not elicit a response; 2) apnea, associated with a change from low- to high-voltage ECoG, increased from 2.2 +/- 1.5 (SE) min in two fetuses at a dose of 0.03 mg to 20 +/- 6.3 min in seven fetuses at 3 mg/kg (P less than 0.005); 3) the length of the breathing responses, associated with a change from high- to low-voltage ECoG, were 15 +/- 1.8 and 135.9 +/- 18.1 min (P less than 0.0005); 4) integral of EMGdi X frequency, an index equivalent to minute ventilation, increased from 1,763 +/- 317 arbitrary units to 10,658 +/- 1,843 at 1.0 mg/kg and then decreased to 7,997 +/- 1,335 at 3.0 mg/kg. These changes were related to a steady increase in integral of EMGdi, whereas frequency decreased at 3 mg/kg. There was an increase in breathing response to morphine plasma concentrations or morphine doses.(ABSTRACT TRUNCATED AT 250 WORDS)     

The incidence of breathing movements of fetal sheep in normoxia and hypoxia after peripheral chemodenervation and brain-stem transection

The incidence of fetal breathing movements and low voltage electrocortical activity was measured in three groups of fetal sheep, at 123-137 days gestation. The first group (transected & denervated) had the brainstem transected at the level of the colliculi and also had peripheral arterial chemodenervation. The second group (denervated) had a sham brain-stem transection and peripheral arterial chemodenervation. The third group (sham-operated) had sham brain-stem transection and sham peripheral chemodenervation. No differences were observed in the incidence of fetal breathing movements or low voltage electrocortical activity between the sham-operated and the denervated groups in normoxia, or in hypoxia when all these fetuses became apnoeic. There were however differences between these 2 groups and the transected & denervated group, in which fetal breathing movements where dissociated from electrocortical activity and which in some fetuses were continuous. During isocapnic hypoxia 3 of 8 transected & denervated fetuses made fetal breathing movements. We discuss the problems of interpreting data from brain-stem transected fetuses, but conclude that the evidence reveals no tonic influence of the peripheral arterial chemoreceptors on fetal breathing movements.     

Adenosine stimulates breathing in fetal sheep with brain stem section.

Breathing responses to adenosine were determined in 12 chronically catheterized fetal sheep (greater than 0.8 term) in which hypoxic inhibition of breathing had been eliminated by brain stem section. The caudal extent of transection varied from the rostral midbrain to the pontomedullary junction. Isocapnic hypoxia [delta arterial PO2 (PaO2) of -12 Torr] doubled the incidence and depth of breathing activity and increased the incidence of eye movements. Intra-arterial infusion of adenosine (0.30 +/- 0.03 mg.min-1.kg fetal wt-1) increased the incidence and amplitude of breathing without affecting blood gases. Adenosine did not significantly alter the incidence of eye activity. Intra-arterial injection of oligomycin (120 +/- 26 micrograms/kg fetal wt), an inhibitor of mitochondrial oxidative phosphorylation, also stimulated breathing activity. In four fetuses with brain stem section, peripheral arterial chemodenervation blunted the stimulatory effects of hypoxia on breathing activity and abolished altogether the excitatory effects of adenosine. It is concluded that 1) hypoxia and adenosine likely inhibit breathing in normal fetuses by affecting similar areas of the brain stem and 2) in fetuses with brain section, hypoxic hyperpnea depends on peripheral and central mechanisms, whereas adenosine stimulates breathing via the peripheral arterial chemoreceptors.     

The effects of gestational age and labour on the breathing and behaviour response to oxygen and umbilical cord occlusion in the fetal sheep.

We tested the hypothesis that the continuous breathing response to oxygen or oxygen plus umbilical cord occlusion, in the fetal sheep, could be modified by gestational age or labour. We studied 35 chronically instrumented fetal sheep on 84 occasions during late gestation (124 to 141 days), using our window model (Rigatto, 1984). After a resting cycle (1 low-voltage followed by 1 high-voltage electrocortical activity epoch), the fetal lung was distended via an endotracheal tube using mean airway pressure of about 30 cm H2O. Inspired nitrogen, and 100% O2 were given to the fetus during one cycle each. While on 100% O2 the umbilical cord was occluded using a balloon cuff. We found that: (1) the continuous breathing response to 100% O2 occurring in 8% of the experiments at a gestational age less than 130 days, in 25% from 130 to 134 days and in 45% at gestational ages greater than 134 days (P < 0.01); (2) at similar gestational age intervals the breathing responses to umbilical cord occlusion were 67%, 84%, and 100% (P < 0.01); and (3) in the presence of labour, 45% of the experiments responded to O2 with continuous breathing as compared to 23% in the absence of labour (P < 0.01). Cord occlusion did not affect these values. Because the highest PaO2 achieved increased significantly to 128 days but not thereafter it is unlikely that these results can be explained on the basis of an increase in PaO2 alone. We speculate that there is an age related maturation of the inhibition of breathing normally present in the fetus.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of naloxone on the changes in breathing and behaviour induced by morphine in the foetal sheep

In the foetal sheep, administration of morphine induces apnoea followed by hyperpnoea; during hyperpnoea the foetus arouses. We tested the hypothesis that naloxone, an opiate antagonist, would block these responses. In 14 foetal sheep between 123 and 140 days of gestation, we measured electrocortical activity (ECoG), eye movements (EOG), diaphragmatic activity (EMGdi), blood pressure and amniotic pressure. Morphine (1 mg/kg) was injected in the foetal jugular vein during low-voltage ECoG. Saline or naloxone (0.1, 0.5 and 2.0 mg) were given, in randomized order, before the morphine injection, shortly after morphine injection during apnoea, and during maximum hyperpnoea. Saline alone had no effect on breathing or behaviour. When saline and naloxone preceded the morphine injection the length of apnoea was 26.6 +/- 7.7 and 19.5 +/- 7.0 min (SEM, P = 0.25) while the length of sustained hyperpnoea was 104.8 +/- 11.4 and 29.6 +/- 8.4 min respectively (P = 0.001). When administered during the maximum breathing response, naloxone decreased the length of breathing from 92.2 +/- 8.4 (saline) to 8.8 +/- 2.9 min (P = 0.001). Respiratory output (fEMGdi x f) also decreased from 6545 +/- 912 arbitrary units post saline to 3841 +/- 629 arbitrary units after naloxone (P = 0.05). Arousal disappeared with the decrease in breathing response. The negligible effect of naloxone on apnoea and its strong inhibition of hyperpnoea suggest that morphine may act on two distinct central regions or on two subtypes of opioid receptors to produce apnoea, hyperpnoea and arousal.     

Effect of beta-endorphin on fetal breathing movements in sheep

It has been suggested that endogenous opioids, such as beta-endorphin (beta-EP), act to depress respiration in the fetus and newborn. We have investigated the effect of infusing beta-EP either intravenously or into a lateral cerebral ventricle on breathing movements and electrocortical activity in eight fetal lambs between 116 and 133 days gestation. Intravenous infusion of beta-EP (200 or 500 micrograms over 1 h) increased plasma beta-EP concentrations 2- to 230-fold and was associated with a small decrease in the percent time spent breathing, from 57.8 +/- 9.1 to 51.3 +/- 8.2%/h (n = 6 exp). There was no change in the amount of high- or low-voltage electrocortical activity. Intracerebroventricular beta-EP infusion (1 or 2 micrograms beta-EP/min for 120 min) was not associated with any change of breathing movements (n = 5 exp) during the period of the infusion. However, in four experiments, in the 6-h period after the end of the beta-EP infusion there were episodes of 2-4 h when the percent time per hour spent breathing exceeded 70%. Electrocortical activity increased in amplitude and distinct episodes of high- and low-voltage activity were sometimes lost in these experiments. We conclude that high concentrations of beta-EP in plasma or cerebrospinal fluid do not totally suppress fetal breathing directly in the fetal lamb.     

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.     

The effects of morphine on the relationship between fetal EEG, breathing and blood pressure signals using fast wavelet transform

In this study, we introduce the fast wavelet transform (WT) as a method for investigating the effects of morphine on the electroencephalogram (EEG), respiratory activity and blood pressure in fetal lambs. Morphine was infused intravenously at 25 mg/h. The EEG, respiratory activity and blood pressure signals were analyzed using WT. We performed wavelet decomposition for five sets of parameters D _2j where -1 < j 5. The five series WTs represent the detail signal bandwidths: 1, 16–32 Hz; 2, 8–16 Hz; 3, 4–8 Hz; 4, 2–4 Hz; 5, 1–2 Hz. Before injection of the high-dose morphine, power in the EEG was high in all six frequency bandwidths. The respiratory and blood pressure signals showed common frequency components with respect to time and were coincident with the low-voltage fast activity (LVFA) EEG signal. Respiratory activity was observed during only some of the LVFA periods, and was completely absent during high-voltage slow activity (HVSA) EEG. The respiratory signal showed dominant power in the fourth wavelet band, and less power in the third and fifth bands. The blood pressure signal was also characterized by dominant power in the fourth wavelet band. This power was significantly increased during periods of respiratory activity. There was a strong relationship between fetal EEG, blood pressure and breathing movements. However, the injection of high-dose morphine resulted in a disruption of the normal cyclic pattern between the two EEG states and a significant increase in power in the first wavelet band. In addition, the high-dose drug resulted in a significant increase in the power of respiratory signal in the fourth and fifth wavelet bands, while power was reduced in the third wavelet band. Breathing activity was also continuous after the drug. The high-dose morphine also caused a temporary power shift from the third wavelet band to the fourth wavelet band for the 30-min period after injection of drug. Finally, high-dose morphine completely destroyed the correlation between EEG, breathing and blood pressure signals.     

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 meclofenamate on breathing movements in fetal sheep before delivery

There is evidence that prostaglandins (PG), specifically PGE2, participate in the regulation of fetal breathing movements (FBM). During late gestation, when FBM occur intermittently and primarily during low-voltage electrocortical activity, the concentration of PGE2 in fetal plasma ([PGE2]) is high. During the days before delivery [PGE2] increases and FBM decrease. To determine whether the increase in [PGE2] is responsible for the concurrent decrease in FBM, we infused the prostaglandin synthase inhibitor, meclofenamate (0.7 mg.kg-1.h-1), into eight fetal sheep continuously for 5-13 days before delivery; five control fetuses received a continuous infusion of the solvent for 5-11 days before delivery. Compared with control infusion, meclofenamate caused a significant decrease in [PGE 2] until the day of delivery and a significant increase in FBM [overall and during high-voltage electrocortical activity (HVA)] until 2 days before delivery. Although there were significant correlations between [PGE2] and FBM (overall and during HVA), both groups showed similar decreases in FBM during the 2 days before delivery. We conclude that the decrease in FBM before delivery is not dependent on the concurrent increase in [PGE2].     

The renal response to the ingestion of fluid by the fetal sheep

To see if the variability in fetal urine flow and sodium excretion was related to fetal drinking activity, renal function was investigated in two groups of oesophageally-ligated fetuses and one group of non-ligated fetuses. There was no significant difference in urine flow, sodium excretion or glomerular filtration rate in the ligated fetuses compared with the non-ligated fetuses. Furthermore, oesophageal ligation had no effect on the variability in urine flow and sodium excretion rate. The response of fetal kidney to ingestion of fluid was investigaeed in 2 groups of oesophageally-ligated fetuses. In one group it was shown that ingestion of 20 ml/kg of amniotic fluid by the fetus had no consistent effect on fetal renal function. In the other group it was shown that the ingestion of 200 ml water also had no consistent effect on fetal renal function. The water load caused a rise in fetal blood pressure and a fall in plasma osmolality. Since there was no significant increase in free water clearance and fetal plasma osmolality decreased then rose towards control levels, it is concluded that the oral water load was absorbed from the fetal gastrointestinal tract and diffused out of the fetal compartment across the placenta. These experiments show that fetal drinking is probably not responsible for the variability often seen in fetal urine flow and sodium excretion rate.     

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.     

Role of plasma adenosine in breathing responses to hypoxia in fetal sheep.

The importance of plasma adenosine in hypoxic inhibition of breathing movements was determined in chronically catheterized fetal sheep (greater than 0.8 term). Preductal arterial blood for adenosine measurements was withdrawn using a double lumen catheter to mix blood entering the catheter with a solution to stop adenosine metabolism. In 6 fetuses, isocapnic hypoxia (delta PaO2 congruent to -10 Torr) increased the average plasma adenosine concentration from 1.1 +/- 0.2 (SEM) to 2.0 to +/- 0.4 microM. During hypoxia, plasma levels of adenosine were inversely related to preductal arterial O2 content (CaO2) with values ranging between 1.6 and 4.0 microM when CaO2 was less than 3 ml/dl. Hypoxia also significantly reduced the incidence of fetal breathing and rapid eye movements. In other experiments, adenosine (0.36 +/- 0.03 mg/min/kg) was infused for one hour into the inferior vena cava of 5 fetuses. During this infusion, mean plasma concentration of adenosine was 2.8 +/- 0.3 microM, a value about 2.5 times the control average. Adenosine also significantly reduced the incidence of low voltage electrocortical activity, rapid eye movements and breathing activity. We conclude that hypoxic inhibition of fetal breathing most likely arises from an increase in central adenosine production, although during severe O2 deprivation (CaO2 less than 3 ml/dl) blood-borne adenosine could also contribute.     

The effects of hypoxia on glucose turnover in the fetal sheep

The origin of the hypoxia-induced rise in fetal blood glucose concentration in fetal sheep of 124-135 days was investigated. Hypoxia was induced in pregnant sheep and fetuses with chronically implanted vascular catheters by causing the ewes to breathe 9% O2 and 3% CO2 in N2 for 60 min. The rise in fetal plasma glucose caused by a 60% reduction in maternal PaO2 was associated with a 50% fall in plasma insulin concentration. The fall in insulin and rise in glucose was prevented by the alpha-adrenergic blocking agent phentolamine but not by the beta-antagonist propranolol. Turnover of glucose in the fetus under these conditions was measured with [6-3H] and [U-14C] glucose. Hypoxia reduced fetal glucose consumption despite the hyperglycaemia. After 30 min of hypoxia there was no evidence of fetal production of glucose but by 60 min substantial production was evident. The reduced fetal consumption and increased production of glucose was inhibited by phentolamine but not by propranolol. It is concluded that in the fetal sheep hypoxia induced hyperglycaemia is first caused by reduced consumption of glucose and thus fetal glycogen stores are not depleted. If the hypoxia persists fetal blood glucose is elevated further by fetal production of glucose.     

Effect of increased arterial CO2 on fetal breathing and behavior in sheep

We studied breathing and behavioral response to increased arterial CO2 (PaCO2) in 12 fetal sheep between 130 and 145 days of gestation. Of these 12 fetuses, 10 had an increase in PaCO2 through maternal rebreathing of CO2; in the other 2 fetuses CO2 was increased via an endotracheal tube and application of continuous distending airway pressure. We used our window technique to observe and videotape fetal behavior. The experiments consisted of recording breathing activity and behavior during resting conditions (1 low- and high-voltage ECoG cycle) and during administration of CO2. We measured electrocortical activity (ECoG), eye movements (EOG), electromyography of the diaphragm (EMGdi) and neck muscles, tracheal (Ptr), amniotic, and carotid arterial pressures. Administration of CO2 by the rebreathing technique produced an increase in the amplitude of breathing activity as reflected by an increase in Ptr from 5.0 +/- 0.6 to 12 +/- 1.9 mmHg (P less than 0.01) and an increase in SEMGdi from 32 +/- 4 to 77 +/- 8% max (P less than 0.001). Frequency increased due to a decrease in inspiratory (TI) and expiratory duration. Ptr/TI increased from 11.0 +/- 2.0 to 37.4 +/- 9.0 mmHg/s (P less than 0.05) and SEMGdi/TI increased from 67 +/- 7 to 221 +/- 28% max/s (P less than 0.001). Although the response was at times prolonged into the transitional high-voltage zone, it did not persist during established high-voltage ECoG.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ontogeny of the insulin response to glucose in fetal and adult sheep

The purpose of the present experiments was to examine in sheep whether the fetal insulin response to glucose was present by day 110 (d110) of pregnancy and whether the magnitude of the fetal insulin response changed between d110 and d145 (term). We also compared the responses observed in fetuses to those of adult nonpregnant sheep. Basal concentrations of glucose measured in plasma collected from the fetal femoral artery rose progressively between d110 and d145 of gestation, but did not attain the plasma glucose concentrations measured in adult sheep. Peak glucose concentrations in fetuses were achieved 10 min following the bolus injection of glucose (0.8 g/kg estimated fetal body weight) into the fetal femoral vein, and peak values increased with gestational age. Significantly higher peak glucose concentrations were achieved in adult sheep. The concentration of insulin rose rapidly in fetuses at d110, and a similar time course of insulin release in plasma was seen at all gestational ages. The peak plasma insulin concentrations were achieved at 20 min and were significantly greater in older (d140-145) than younger (d125-130) fetuses (p less than 0.05). Peak insulin values in fetuses were much less than in adult sheep. In adult sheep glucose and insulin concentrations remained elevated at 120 min following the injection of glucose, whereas in the fetus the concentration of insulin had returned to preinjection values by 60 min. The insulin/glucose ratio did not change in fetal lambs over the last one third of gestation and was not different from the adult sheep.(ABSTRACT TRUNCATED AT 250 WORDS)