Metabolic and cardiorespiratory responses to hypoxia in fetal sheep: adenosine receptor blockade

8-Phenyltheophylline (PT), a potent and specific inhibitor of adenosine receptors, was infused intra-arterially into unanesthetized fetal sheep to determine the role of adenosine in hypoxic inhibition of fetal breathing. PT in normoxic fetuses increased heart rate and the incidence of low-voltage electrocortical activity, rapid eye movements (REM), and breathing. Mean breath amplitude increased by 44%. Hypoxia (preductal arterial PO2 = 14 Torr) induced a metabolic acidemia, a transient bradycardia, and hypertension while virtually eliminating REM and breathing. PT administration during hypoxia enhanced the metabolic acidemia, blocked the bradycardia and hypertension, increased the incidence of REM and breathing, and elevated mean breath amplitude. The results indicate that 1) adenosine is involved in fetal glycolytic and cardiovascular responses to hypoxia, 2) activation of central adenosine receptors mediates about one-half the inhibitory effects of hypoxia on REM and breathing, and 3) the depression of breathing may critically depend on a hypoxia-induced reduction in phasic REM sleep.

NMDA receptor-mediated calcium influx in cerebral cortical synaptosomes of the hypoxic guinea pig fetus

Calcium influx via the NMDA receptor has been proposed as a mechanism of hypoxia-induced neuronal injury. The present study tests the hypothesis that the increase of [Ca2+]i observed under hypoxic conditions is the result of an NMDA-mediated Ca2+ influx. Changes of [Ca2+]i, measured fluorometrically with Fura-2, were followed after activation of the NMDA receptor with NMDA and glutamate, in the presence of glycine, in cortical synaptosomes prepared from six normoxic and six hypoxic guinea pig fetuses. [Ca2+]i was significantly higher in hypoxic vs normoxic synaptosomes, at baseline and in the presence of glycine as well as following activation of the NMDA receptor. Increase in [Ca2+]i was not observed in a Ca2+ free medium and was significantly decreased by MK-801 and thapsigargin. These results demonstrate that hypoxia-induced modifications of the NMDA receptor ion-channel results in increased [Ca2+]i in hypoxic vs normoxic synaptosomes. This increased accumulation may be due to an initial influx of Ca2+ via the altered NMDA receptor with subsequent release of Ca2+ from intracellular stores. Increase in intracellular calcium may initiate several pathways of free radical generation including cyclooxygenase, lipoxygenase, xanthine oxidase and nitric oxide synthase, and lead to membrane lipid peroxidation resulting in neuronal cell damage.

Are maternal diabetes and preeclampsia independent simulators of fetal erythropoietin production?

To determine if diabetes and preeclampsia are independent stimulators of erythropoietin, distinct from hypoxia, we measured umbilical cord plasma erythropoietin in 239 deliveries from 24 to 40 weeks of gestation. Mean plasma erythropoietin levels were not different between normal, diabetic, and preeclamptic women when all deliveries were analyzed. When infants with suspected intrauterine hypoxia were excluded, the mean erythropoietin level was considerably lower within all three groups but there was no difference among the groups. In suspected hypoxia, the mean fetal erythropoietin was elevated, but there was no difference between control, diabetic, or preeclamptic pregnancies. These results provide further support that hypoxia remains the only known stimulator of erythropoietin production in the fetus.

Gestational age-dependent changes in the levels of mRNAs encoding cortisol biosynthetic enzymes and IGF-II in the adrenal gland of fetal sheep during prolonged hypoxemia

Hypoxemia represents a major stress for the fetus, and is associated with alterations and adaptations in cardiovascular, metabolic and endocrine responses, which in turn may affect tissue growth and differentiation. To determine the effects of hypoxemia on fetal adrenal activity and growth, we subjected sheep fetuses at days 126-130 and 134-136 (term 145 days) to reduced PaO2 by reducing the maternal fraction of oxygen for 48 h (mean reduction of 6.8 mmHg), without change in arterial pH or PaCO2. This stimulus resulted in similar increases in the plasma immunoreactiveACTH response at both ages. Among adrenal steroids, plasma cortisol (C21Delta4) rose in both groups of animals, but plasma androstenedione (C19Delta4) declined marginally, resulting in a pronounced increase in the cortisol:androstenedione ratio in the plasma that was greater and more sustained in the older fetuses. In the younger fetuses, after 48 h of hypoxemia, there were no significant changes in mRNAs encoding steroidogenic enzymes in the fetal adrenal gland. However, in the older fetuses, hypoxemia resulted in significantly increased levels of mRNAs encoding P450scc, P450C21 and 3beta-hydroxysteroid dehydrogenase, but not for P450C17, in the fetal adrenal gland. Levels of IGF-II mRNA in the fetal adrenal gland fell in both groups of fetuses, and this response was greater at the later gestational age. We conclude that sustained hypoxemia is a potent stimulus which activates adrenal steroidogenesis in the late gestation fetal sheep. The resultant increase in cortisol synthesis is associated with decreased expression of adrenal IGF-II mRNA. We speculate that this relationship might influence patterns of fetal organ growth and differentiative function in response to fetal stress such as hypoxemia.

Proton magnetic resonance spectroscopy of fetal lamb brain during hypoxia

Proton magnetic resonance spectroscopy of fetal lamb brain was performed simultaneously with repeated measurements of fetal arterial oxygen saturation during decrease of oxygen supply. Magnetic resonance spectra displayed the same metabolite peaks as detected in the human fetal brain. Cerebral lactate signals could be detected during fetal hypoxia.

Intracardiac iron distribution in newborn guinea pigs following isolated and combined fetal hypoxemia and fetal iron deficiency

Myocardial iron deficiency complicates chronic intrauterine hypoxemia during diabetic pregnancies. To understand the effect of both conditions during fetal life on intracardiac iron prioritization, we measured heart myoglobin, cytochrome c, and elemental iron concentrations in six iron-deficient, hypoxic, five iron-sufficient, hypoxic, six iron-deficient, normoxic, and six iron-sufficient, normoxic newborn guinea pigs. The iron-deficient, hypoxic group had lower heart iron (p = 0.03) but higher myoglobin concentration (p < 0.0001) when compared with the iron-sufficient, normoxic group. The percentage of iron incorporated into myoglobin was higher than control in the iron-deficient, hypoxic group (23.2+/-7.2% vs. 5.2+/-0.8%; p < 0.001) and increased as total heart iron decreased (r = 0.97; p < 0.001). In contrast, heart cytochrome c concentration was lower than control in the iron-deficient, hypoxic group (p = 0.01), with equal percentages of heart iron incorporated into cytochrome c. This intracellular prioritization of myocardial iron to myoglobin and away from cytochrome c following combined fetal hypoxemia and iron deficiency may represent an adaptive mechanism to preserve myocardial tissue oxygenation, although at the expense of oxidative phosphorylative capability.

Fetal endocrine responses to prolonged reduced uterine blood flow are altered following bilateral sectioning of the carotid sinus and vagus nerves

The present study examines the effect of carotid sinus/vagosympathetic denervation on fetal endocrine responses to prolonged reduced uterine blood flow (RUBF). Fetal sheep had vascular catheters inserted following bilateral sectioning of the carotid sinus and vagus nerves (denervated, n = 7) or sham denervation (intact, n = 7). Uterine blood flow was mechanically restricted at 126.1 +/- 0.7 days (mean +/- S.E.M.) for 24 h, decreasing arterial oxygen saturation by 47.3 +/- 2.6% (P < 0.01). Fetal plasma samples were obtained at -1, 3, 6, 12 and 24 h for subsequent analyses of arginine vasopressin (AVP), angiotensin II and catecholamines. The AVP response to prolonged RUBF was markedly attenuated in denervated fetuses (15.6 +/- 3.6 to 34.9 +/- 6.0 pg/ml) when compared with intact (10.0 +/- 1.4 to 127.3 +/- 28.4 pg/ml). In contrast, intact fetuses demonstrated no change in plasma angiotensin II concentrations with RUBF whereas denervated fetuses demonstrated a marked increase from 47.5 +/- 18.9 to 128.7 +/- 34.2 pg/ml. The norepinephrine and epinephrine responses to prolonged RUBF were attenuated in denervated fetuses (950.1 +/- 308.9 and 155.8 +/- 58.5 to 1268.3 +/- 474.6 and 290.6 +/- 160.2 pg/ml respectively) when compared with intact (1558.3 +/- 384.4 and 547.3 +/- 304.7 pg/ml to 3289.2 +/- 1219.8 and 896.8 +/- 467.8 pg/ml respectively). These results support a role for the peripheral chemoreceptors in mediating fetal endocrine responses to prolonged RUBF, which may in part lead to the altered cardiovascular responses observed in denervated fetuses under these conditions.

Metabolic and circulatory adaptations to chronic hypoxia in the fetus

When oxygenation is compromised the fetus is capable of a number of adaptive responses, both protective and potentially pathologic, which can be categorized as those affecting fetal metabolism and those affecting fetal oxygen transport. However, both the extent and the duration of the impairment in oxygenation will bear on these adaptive responses. While fetal O2 extraction is increased when oxygenation is acutely compromised thus maintaining O2 consumption, with chronic hypoxemia there is a decrease in O2 consumption paralleling that in O2 delivery and contributed to by the resultant fall-off in growth and alterations in behavioural activity. While a redistribution of blood flow to vital organs continues to be evident, this will be less pronounced than that seen with acute hypoxemia reflecting diminished hormonal changes, underlying metabolic alterations, and the extent to which fetal blood gases are normalized. Much of this information is based on experimental data using unanesthetized fetal sheep with chronic catheterization; however, clinical outcome data and the use of investigative techniques including ultrasound scanning and cordocentesis have supported the relevance of this experimental data to the human situation.

Fetal myocardial responses to long-term hypoxemia

In fetal sheep subjected to high altitude hypoxemia for 110 days beginning on day 30 of gestation, cardiac output was decreased 24% compared to normoxic control fetuses. This decrease was due to a 33% reduction in right ventricular output, with only a 14% reduction in left ventricular output. There were no changes in preload or heart rate, but approximately 7% of the reduction in cardiac output could be explained by an increase in arterial blood pressure (afterload). In papillary muscle isolated from long-term hypoxemic fetal hearts, maximum developed tension in response to increasing concentrations of calcium was reduced in both the right and left ventricles, but sensitivity to calcium was increased in both. This finding suggests alterations in the calcium pathway for excitation-contraction coupling in the hypoxemic fetal hearts may be responsible for the reduction in contractility. The mechanism for the decrease in contractility could not be explained by changes in sarcolemmal L-type calcium channel number or sarcoplasmic reticulum calcium release channel number. In addition, there were no changes in the calcium-induced calcium release mechanism involving the sarcoplasmic reticulum, which could explain the reduced contractility. We speculate that the decreased calcium response may be due to other factors, such as the amount of calcium stored in the sarcoplasmic reticulum, myofilament calcium sensitivity, or cellular content of myofilaments.

Brain lipid peroxidation and antioxidant levels in fetal lambs 72 hours after asphyxia by partial umbilical cord occlusion

OBJECTIVES

The purpose of this study was to explain the role of oxidative stress in the pathogenesis of brain damage caused by intrauterine fetal asphyxia.

STUDY DESIGN

Six chronically instrumented near-term fetal lambs were subjected to asphyxia by partial umbilical cord occlusion for approximately 60 minutes until fetal arterial pH diminished to less than 6.9 and base excess to less than -20 mEq. Another six fetuses surgically prepared but not occluded were used as control. Fetuses were killed after 72 hours and eight different brain regions (frontal and parietal gray matter, frontal and parietal white matter, basal ganglia, thalamus, hippocampus, and cerebellum) were dissected and assayed for thiobarbituric acid reactive substances, glutathione, and superoxide dismutase.

RESULTS

Thiobarbituric acid reactive substance levels in asphyxiated animals were elevated in frontal and parietal white matter, basal ganglia, and thalamus compared with those in controls. The concentrations of superoxide dismutase in the asphyxiated group were also higher in frontal and parietal white matter, basal ganglia, and cerebellum compared with those in the control group. Between the two groups, however, glutathione concentrations did not differ significantly.

CONCLUSION

These results suggest that oxidative stress may be a major contributing factor to the development of brain damage in intrauterine fetal asphyxia.

Erythropoietin in amniotic fluid as a marker of chronic fetal hypoxia

OBJECTIVE

To determinate the erythropoietin concentration in amniotic fluid in normal pregnancies and pregnancies with suspected hypoxia.

METHOD

The erythropoietin concentration of 164 samples of amniotic fluid was determined by ELISA. The samples were taken by amniotomy during birth, as well as amniocentesis conducted during prenatal care.

RESULTS

A distribution of 1.07-7.29 U/l (10th-90th percentile) within the normal group (n = 106) was determined. Significantly elevated erythropoietin levels in amniotic fluid were determined in maternal hypertension (P = 0.039) and low birth-weight children (P = 0.0032). A correlation with the child's sex could be excluded.

CONCLUSION

Elevated erythropoietin levels in amniotic fluid indicated chronic fetal hypoxia.

Chronic hypoxemia causes extracellular glutamate concentration to increase in the cerebral cortex of the near-term fetal sheep

Fetal hypoxia is an important cause of neurologic morbidity and mortality. Hypoxia-induced increase in extracellular glutamate concentration can lead to excitotoxic neuronal death in adults. The objective of this study was to test whether chronic fetal hypoxemia increases extracellular glutamate concentration in the unanesthetized intact cerebral cortex of the near-term fetal sheep. Microdialysis probes were implanted into the parasagittal parietal cortex and periventricular white matter of near-term fetal sheep. At 124 +/- 1 days of gestation, extracellular glutamate concentration was determined before and during 24 h of fetal hypoxemia. Chronic hypoxemia was produced by tightening a vascular occluder placed around the maternal common iliac artery. Larger decreases in fetal arterial oxygen content were associated with larger increases in extracellular glutamate concentration in the parietal cortex (Kendall's tau = 0.81, N = 7, p = 0.005). No such relationship was detected in the periventricular white matter. Chronic hypoxemia increases extracellular glutamate concentration in the intact cerebral cortex of the unanesthetized near-term fetal sheep.

Changes in purine metabolism and production of oxygen free radicals by intermittent partial umbilical cord occlusion in chronically instrumented fetal lambs

OBJECTIVE

Undetected umbilical-cord compression has been suggested to be implicated in unexplained fetal brain damage. We tested the hypothesis that the generation of oxygen-free radicals (OFRs) during intermittent partial umbilical cord occlusion might play a causal role in antenatal CNS injury.

METHODS

Using 7 established chronically instrumented fetal lambs, intermittent partial occlusion of the umbilical circulation was produced according to the method of Clapp et al. for 1 of every 3 minutes for 2 hours. A microdialysis probe was implanted in the fetal brain white matter. Normal saline was infused, and the perfusate obtained at 2 micrograms/ min. Hypoxanthine (HX), xanthine (XA), and inosine-5-monophosphate (IMP) concentrations in the perfusate and blood samples obtained from fetal jugular veins were assayed by HPLC. Concurrently, the perfusate, which contained superoxide produced in the brain, when mixed with cypridina luciferin analogue extracorporally, caused chemiluminescence that in 4 cases was measured by a highly sensitive electronic fluorescence detector.

RESULTS

(1) HX, XA, and IMP concentrations in the blood and perfusate were higher than in the control during the intermittent partial occlusion of the umbilical circulation period and returned to control levels during the recovery period. (2) The residual chemiluminescence of perfusate revealed that the intermittent partial occlusion of the umbilical circulation level was about 100% higher than the control level, and during the recovery period the level returned to the control level. (3) The chemiluminescence during the intermittent partial occlusion of the umbilical circulation period was inhibited by infusion through the fetal jugular vein of 1 micron polyethylene glycol conjugated superoxide dismutase.

CONCLUSION

During the intermittent partial occlusion of the umbilical circulation period the fetal brain tissue releases a large amount of OFRs, and a portion of these might be synthesized by the increased conversion of HX to XA. This phenomenon might play an important role in the etiology of fetal brain injury.

Source of extracellular brain adenosine during hypoxia in fetal sheep

Microdialysis was performed to determine whether hypoxia increases fetal brain adenosine (ADO) concentration through dephosphorylation of extracellular 5'-adenosine monophosphate (5-AMP). Hypoxia (fetal PaO2 approximately 14 Torr) increased fetal brain ADO levels approximately two-fold when the probes were perfused with synthetic cerebrospinal fluid (CSF) containing inhibitors of the nucleoside transporter but not with this solution plus a blocker of ecto-5'-nucleotidase (AOPCP). The hypoxia-induced rise in fetal brain ADO concentrations depends critically upon the hydrolysis of extracellular 5'-AMP.

Oxygen and placental villous development: origins of fetal hypoxia

The increasing practice of preterm delivery in the fetal interest for conditions such as pre-eclampsia or intrauterine growth restriction (IUGR) has provided an opportunity to study placental structure in pregnancies with prenatal evidence of fetal compromise. These data suggest that the origin of fetal hypoxia in IUGR with absent end-diastolic flow in the umbilical arteries is due to a failure of oxygen transport from intervillous space to umbilical vein. Failure of the fetoplacental circulation to extract oxygen from the intervillous space under such circumstances means intervillous PO2 is closer to maternal arterial values than under physiological conditions. Correspondingly the placental villi are chronically exposed to a higher oxygen tension than under normal circumstances--the term ¿hyperoxia', relative to normal intraplacental oxygenation, is proposed to describe this situation. Both the trophoblast and villous core react to increased oxygen despite fetal hypoxia. These results challenge the generally accepted concept of ¿placental hypoxia' in all circumstances where fetal hypoxia might arise. Therefore three categories are proposed for the origins of fetal hypoxia: (1) preplacental hypoxia; (2) uteroplacental hypoxia; and (3) postplacental hypoxia. Examples for these three disease states are listed in this review and the structural reaction patterns of placental villi to these differences in oxygenation are discussed.

Developmental regulation of preproenkephalin (PENK) gene expression in the adrenal gland of the ovine fetus and newborn lamb: effects of hypoxemia and exogenous cortisol infusion

Development of the fetal adrenal gland is crucial not only for maturation of several fetal organ systems and the initiation of parturition, but also for the development of the fetal response to stress. The enkephalin-related peptides are present in the chromaffin cells of the fetal adrenal medulla and are secreted in response to stress and with sympathetic stimulation. However, changes in expression of preproenkephalin (PENK) with gestation and in response to stress have not been studied in detail. Therefore we examined the developmental pattern of PENK gene expression in the adrenal gland of fetal and newborn lambs, and of adult sheep. We also determined whether levels of PENK mRNA in the fetal adrenal gland changed in response to exogenous glucocorticoids in late gestation, or in response to hypoxemia. Adrenal glands were removed from fetal sheep, lambs and adult sheep at different stages of development for measurement of PENK mRNA. Cortisol was infused (5 micrograms/min) for 12, 24 or 96 h beginning on day 124-129 of gestation. Moderate hypoxemia was induced for 48 h beginning on day 126-130, or at day 134-136 of gestation, by lowering the maternal fractional inspired oxygen. At the end of the treatment periods, the ewes and fetuses were euthanized. Adrenal PENK mRNA were measured by Northern blot analysis. PENK mRNA levels in fetal adrenals were significantly higher (P < 0.05) on days 140-141 of gestation than earlier in pregnancy, and then decreased significantly with the onset of parturition (days 142-146). After cortisol infusion to the fetus for 96 h there was a significant reduction in adrenal PENK mRNA levels. Hypoxemia resulted in a significant increase in PENK mRNA levels in fetuses at day 126-130 of gestation, but not at the later time in pregnancy when endogenous plasma cortisol concentrations were higher. We conclude that there is a decrease in levels of PENK mRNA in the fetal adrenal gland before parturition at the time of the endogenous prepartum rise in plasma cortisol. Hypoxemia led to an elevation of PENK mRNA levels in fetuses at less than 130 days, but after that time, when the basal and stimulated cortisol responses had risen, there was no significant effect of hypoxemia on PENK mRNA. Cortisol infusion to the fetus at this stage of pregnancy resulted in a decrease in adrenal PENK mRNA levels. We suggest that cortisol may play an important role in the regulation of fetal adrenal PENK mRNA levels and enkephalin synthesis by the adrenal gland of the fetal sheep.

Effect of perinatal asphyxia on systemic and intracerebral pH and glycolysis metabolism in the rat

The effects of perinatal asphyxia on systemic and brain pH and glycolysis metabolism were studied in the rat. Perinatal asphyxia was induced by immersing pup-containing uterus horns, obtained by cesarean section from rats within the last day of gestation, in a water bath at 37 degrees C for various periods of time (0-23 min). Subcutaneous levels of pyruvate (Pyr), lactate (Lact), glutamate (Glu), and aspartate (Asp) were monitored with microdialysis 40-80 min after delivery. In parallel experiments, the pups were sacrificed 40 min after delivery and the heart and brain were removed for measuring pH. Brain (striatum) Pyr, Lact, Glu, and Asp levels were also analyzed. A decrease in the rate of survival was first observed following asphyctic periods longer than 16 min, and no survival could be observed after 22 min of asphyxia. In control (cesarean-delivered) pups, heart and brain pH were 7.36 +/- 0.01 (N = 8) and 7.30 +/- 0.01 (N = 8), respectively. Significant decreases in pH were first observed following 5-6 and 10-11 min of asphyxia, in heart and brain, respectively. In both regions pH decreased along with the length of asphyxia, but a decrease below 7 was only observed in the brain, following asphyctic periods longer than 16 min. A significant increase in subcutaneous Lact levels was first observed following 2-3 min of asphyxia, with a maximum after 20-21 min of asphyxia. In the brain, the increase in Lact levels was delayed compared to that observed in subcutaneous tissue. Pyr and Asp levels increased in subcutaneous tissue following perinatal asphyxia and decreased in brain tissue following > 15 min of asphyxia. Glu levels were increased subcutaneously by moderate (5-16 min) asphyctic periods, but, in the brain, were only transiently increased by 10-11 min of asphyxia. Thus, changes in systemic pH, glycolysis, and excitatory amino acid metabolism are observed following shorter asphyctic periods than are changes in the brain. In particular, increases in subcutaneous Lact levels precede: (i) a decrease in brain pH, (ii) an increase in brain Lact levels, (iii) a decrease in the rate of survival, and, probably, (iv) brain damage. It is suggested that monitoring Lact levels by subcutaneous microdialysis is a useful method for predicting the outcome produced by hypoxic-ischemic insults.

Long-term reciprocal changes in dopamine levels in prefrontal cortex versus nucleus accumbens in rats born by Caesarean section compared to vaginal birth

Epidemiological evidence indicates a higher incidence of pregnancy and birth complications among individuals who later develop schizophrenia, a disorder linked to alterations in mesolimbic dopamine (DA) function. Two birth complications usually included in these epidemiological studies, and still frequently encountered in the general population, are birth by Caesarean section (C-section) and fetal asphyxia. To test the hypothesis that birth complications can produce long-lasting changes in DA systems, the present study examined the effects of Caesarean birth, with or without an added period of anoxia, on steady state monoamine levels and metabolism in various brain regions in a rat model. Pups born vaginally served as controls. At 2 months of age, in animals born by rapid C-section, steady state levels of DA were decreased by 53% in the prefrontal cortex and increased by 40% in both the nucleus accumbens and striatum, in comparison to the vaginally born group. DA turnover increased in the prefrontal cortex, decreased in the nucleus accumbens, and showed no significant change in the striatum, in the C-section group. Thus, birth by a Caesarean procedure produces long-term reciprocal changes in DA levels and metabolism in the nucleus accumbens and prefrontal cortex. This is consistent with the known inhibitory effect of increased prefrontal cortex DA activity on DA release in the nucleus accumbens. By contrast to birth by rapid C-section alone, young adult animals, that had been born by C-section with 15 min of added anoxia, showed no change in steady state DA levels in the prefrontal cortex, nucleus accumbens, or striatum and a significant decrease in DA turnover only in the nucleus accumbens, in comparison to the vaginally born group. Levels of norepinephrine, serotonin, and its metabolite, 5-hydroxyindole acetic acid, were unchanged in all groups, indicating relatively specific effects on DA systems. Although appearing robust at birth on gross observation, more subtle measurements revealed that rat pups born by C-section show altered respiratory rates and activity levels and increased levels of whole brain lactate, suggestive of low grade brain hypoxia, during the first 24 h of life, in comparison to vaginally born controls. Pups born by C-section with 15 min of added acute anoxia were pale, hypotonic, and inactive at birth and showed reduced respiration and high brain lactate levels. However, these alterations resolved by 1-5 h after birth and, with few exceptions, animals in the anoxic group remained normal with respect to these parameters during the remainder of the first 24 h of life. Immediately after birth, levels of plasma epinephrine, a hormone known to play a role in neonatal adaptation to extrauterine life and protection against hypoxia, were decreased in pups born by C-section but increased in pups born by C-section with 15 min added anoxia, in comparison to levels measured in vaginally born controls. These early developmental alterations could contribute to long-term alterations in dopaminergic parameters observed in rats born by C-section, with or without added anoxia. It is concluded that C-section birth is sufficient perturbation to produce long-lasting effects on DA levels and metabolism in the central nervous system of the rat. These findings highlight the sensitivity of DA pathways to variations in birth procedure and support the notion that birth complications might contribute to the pathophysiology of disorders involving central dopaminergic neurons, such as schizophrenia.

Influence of perinatal factors on the nucleus accumbens dopamine response to repeated stress during adulthood: an electrochemical study in the rat

Evidence from animal studies suggests that a period of anoxia to the fetus, a consequence common to many birth complications, results in long-term alterations in ventral mesencephalic dopamine function. Long-term functional changes in these dopamine neurons, in particular those that innervate the nucleus accumbens, also occur when animals are repeatedly stressed. In the present study, we examined the possibility that a period of anoxia during a Cesarean section birth can later alter the development of stress-induced sensitization of dopamine transmission in the nucleus accumbens. Dams were decapitated on the last day of gestation and the entire uterus was removed by Cesarean section. Pups were then delivered either immediately (Cesarean section group) or were immersed in a 37 degrees C saline bath for 3.5 or 13.5 min (Cesarean section+anoxia groups) before delivery of the pups. A fourth group of pups that were born vaginally served as controls (Vaginal group). Three to four months postnatally, animals from each group were implanted with monoamine-selective carbon-fiber electrodes into the nucleus accumbens. Voltammetry was used to monitor the dopamine response to each of five consecutive, once daily, 15-min exposures to tail-pinch stress. The results show that the first exposure to stress elicited dopamine signal increases of comparable amplitudes and durations in all animals. However, when compared to the initial stress response, the fourth and fifth exposures to tail-pinch elicited significantly longer-lasting dopamine responses in animals born by Cesarean section, either with or without added anoxia. In contrast, there was no significant day-to-day enhancement of the stress response in control, vaginally born animals. The findings reported here provide experimental support for the idea that birth complications may contribute to the pathophysiology of psychiatric disorders, in particular those that involve central dopamine dysfunction, such as schizophrenia. Specifically, our results suggest that subtle alterations in birth procedure may be sufficient to increase the sensitivity of mesolimbic dopamine neurons to the effects of repeated stress in the adult animal.