Adverse respiratory patterns in near-term spontaneously breathing newborn lambs with elevated airway liquid volumes at birth

Introduction

Recent evidence indicates that respiratory distress (RD) in near-term infants is caused by elevated airway liquid (EL) volume at the beginning of air-breathing after birth. While the adverse effects EL volumes on newborn lung function are known, the effects on respiratory control and breathing patterns shortly after birth (<4 h) are unknown. We investigated the effects of EL volumes on cardiorespiratory function and breathing patterns in spontaneously breathing near-term newborn lambs in the first hours after birth.

Methods

At 137-8 days gestation (2-3 days prior to delivery; term ∼147 days), sterile surgery was performed on fetal sheep (n = 17) to implant catheters and blood flow probes. At 140 days, lambs were delivered via caesarean section under spinal anaesthesia. Airway liquid volumes were adjusted to mimic the level expected following vaginal delivery (∼10 ml/kg; Controls; n = 7), or elective caesarean section (∼30 ml/kg; elevated airway liquid group; EL; n = 10). Spontaneous breathing and cardiorespiratory parameters were recorded over four hours after birth. Non-invasive respiratory support with supplemental oxygen was provided if required.

Results

EL lambs required higher inspired oxygen levels (p = 0.0002), were less active (p = 0.026), fed less (p = 0.008) and had higher respiratory morbidity scores than Controls (p < 0.0001). EL lambs also displayed higher rates of breathing patterns associated with RD, such as expiratory braking and tachypnoea. These patterns were particularly evident in male EL lambs who displayed higher levels of severe respiratory morbidity (e.g., expiratory braking) than female EL lambs.

Conclusion

The study demonstrates that EL volumes at birth trigger respiratory behaviour and breathing patterns that resemble clinically recognised features of RD in term infants.

Respiratory patterns in spontaneously breathing near-term lambs delivered by caesarean section under spinal anaesthesia

Introduction

The transition to newborn life has typically been studied in intubated and mechanically ventilated newborn lambs delivered via caesarean section (CS) under general anaesthesia. As a result, little is known of the spontaneous breathing patterns in lambs at birth, particularly those at risk of developing respiratory distress (RD). We have developed a method for delivering spontaneously breathing near-term lambs to characterise their breathing patterns in the immediate newborn period.

Methods

At 137-8 days gestation (2-3 days prior to delivery; term ∼147 days), fetal lambs (n = 7) were partially exteriorised for instrumentation (insertion of catheters and flow probes) before they were returned to the uterus. At 140 days, lambs were delivered via CS under light maternal sedation and spinal anaesthesia. Lambs were physically stimulated and when continuous breathing was established, the umbilical cord was clamped. Breathing patterns were assessed by measuring intrapleural and upper-tracheal pressures during the first four hours after birth.

Results

Newborn lambs display significant heterogeneity in respiratory patterns in the immediate newborn period that change with time after birth. Seven distinct breathing patterns were identified including: (i) quiet (tidal) breathing, (ii) breathing during active periods, (iii) breathing during oral feeding, (iv) tachypnoea, (v) expiratory braking manoeuvres, (vi) expiratory pauses or holding, and (vii) step changes in ventilation.

Conclusions

We have described normal respiratory behaviour in newborn lambs, in order to identify respiratory behaviours that are indicative of RD in term newborn infants.

Assessing lung aeration using ultrasound after birth in near-term lambs at risk of respiratory distress

Background

Optimizing respiratory support after birth requires real-time feedback on lung aeration. We hypothesized that lung ultrasound (LUS) can accurately monitor the extent and progression of lung aeration after birth and is closely associated with oxygenation.

Methods

Near-term (140 days gestation, term ∼147 days), spontaneously breathing lambs with normal (controls; n = 10) or elevated lung liquid levels (EL; n= 9) were delivered by Caesarean section and monitored for four hours after birth. LUS (Phillips CX50, L3-12 transducer) images and arterial blood gases were taken every 5-20 min. LUS images were analyzed both qualitatively (grading) and quantitatively (using the coefficient of variation of pixel intensity (CoV) to estimate the degree of lung aeration), which was correlated with the oxygen exchange capacity of the lungs (Alveolar-arterial difference in oxygen; AaDO₂).

Results

Lung aeration, measured using LUS, and the AaDO₂ improved over the first 4 h after birth. The increase in lung aeration measured using CoV of pixel intensity, but not LUS grade, was significantly reduced in EL lambs compared to controls (p = 0.02). The gradual decrease in AaDO₂ after birth was significantly correlated with increased lung aeration in both control (grade, r²= 0.60, p < 0.0001; CoV, r²= 0.54, p < 0.0001) and EL lambs (grade, r²= 0.51, p < 0.0001; CoV, r²= 0.44, p < 0.0001).

Conclusions

LUS can monitor lung aeration and liquid clearance after birth in spontaneously breathing near-term lambs. Image analysis techniques (CoV) may be able detect small to moderate differences in lung aeration in conditions with lung liquid retention which are not readily identified using qualitative LUS grading.

Intraperitoneal medetomidine: A novel analgesic strategy for postoperative pain management in pregnant sheep

The absorption of medetomidine released by continuous infusion from an osmotic pump in the abdominal cavity was studied in pregnant sheep during the 24 h postoperative period. Additionally pain and sedation was assessed. Eleven sheep were studied: six were treated with a medetomidine loaded osmotic pump delivering 10 μL/h (3 μg/kg/h medetomidine); and five with a saline loaded osmotic pump (control). Serial blood samples were taken and analysed to determine plasma medetomidine levels. Medetomidine was absorbed from the peritoneal cavity and a steady plasma concentration was achieved within 10 h, mean (SD) peak concentration was 2.87 (0.22) ng/mL. Sheep receiving medetomidine analgesia had significantly lower pain scores at 10 h than controls. Four control sheep required rescue analgesia, compared with 0 in the treatment group. Delivery of 3 μg/kg/h medetomidine by an intraperitoneal osmotic pump to pregnant sheep in the 24 h postoperative period provides adequate plasma concentrations of medetomidine for analgesia without sedation.

The cerebral critical oxygen threshold of ventilated preterm lambs and the influence of antenatal inflammation

Perinatal inflammation is associated with adverse neurodevelopmental outcomes, which may be partly due to changes in the cerebral oxygen delivery/consumption relationship. We aimed to determine the critical oxygen delivery threshold of the brain of preterm, ventilated lambs and to determine whether the critical threshold is affected by exposure to inflammation in utero. Pregnant ewes received intra-amniotic injection of lipopolysaccharide or saline at 125 or 127 days of gestation. Pulmonary and systemic flow probes and catheters were surgically positioned in the fetus immediately before delivery at 129 days of gestation. After delivery, lambs were ventilated for 90 min using a positive end-expiratory pressure recruitment strategy. Cardio-respiratory variables and blood gases were measured regularly. Systemic and cerebral oxygen delivery, consumption (Fick), and extraction were calculated, and the relationship between cerebral delivery and consumption analyzed. Linear regression was used to define the transition or "critical" oxygen threshold as the point at which the slope of the oxygen delivery/consumption curve changed to be > 10°. Four subgroups were defined according to the calculated critical threshold. A total of 150 measurements were recorded in 18 lambs. Fetal cerebral oxygen consumption was increased by antenatal lipopolysaccharide (P < 0.05). The postnatal critical oxygen threshold was 3.6 ml·kg⁻¹·min⁻¹, corresponding to cerebral oxygen consumption of 0.73 ml·kg⁻¹·min⁻¹. High oxygen delivery and consumption were associated with increased pulmonary and carotid blood flow and systemic extraction compared with low oxygen delivery and consumption. No postnatal effect of antenatal inflammation was observed. Inflammation in utero increases fetal, but not postnatal, cerebral oxygen consumption. Adverse alterations to pulmonary blood flow can result in reduced cerebral blood flow, oxygen delivery, and consumption. Regardless of exposure to inflammation, there is a consistent postnatal relationship between cerebral oxygen delivery and consumption.

007.Recent advances in understanding and preventing pre-term birth - the oral health connection

Pre-term birth remains one of the major unsolved problems in human health. The incidence is increasing in many Western countries, despite several decades of research studies aimed at findings ways that early birth may be prevented. Nearly two-thirds of very early pre-term births are associated with features of inflammation in the newborn, suggesting that infection may be the origin. Many studies have focussed on the possibility that pre-labour rupture of membranes or the early onset of uterine activity may result from infection spreading upward from the vagina. Unfortunately, trials designed to identify potential pathogens in the genital tract followed by appropriate use of antibiotics have failed to prevent prematurity. The strong association between features of intra-uterine inflammation and pre-term birth, and the ineffectiveness of antibiotics to prevent the problem, suggest that the source of inflammation may be from a distant site. We are addressing the possibility that the site may be the gums. Periodontal disease affects 15% of the adult Australian population, is often undiagnosed and is not responsive to systemic antibiotic therapy. In our pregnant population, we have shown that periodontal disease is strongly associated with low birth weight. Our sheep studies have taught us that the lipopolysaccharides (LPS) from periodontal pathogens, when injected into the amniotic cavity, have much greater lethality than enteric LPS, and similar effects in inducing inflammation. We are now investigating the effects of treating periodontal disease during mid-pregnancy in a randomised controlled trial, which aims to screen approximately 5000 pregnant women and allocates those with periodontal disease to treatment during pregnancy or soon after. This study is known as the Smile Study and commenced in February 2005. Improving oral health is an exciting, but yet unproven, strategy by which a major health problem may be prevented by a relatively simple and community-based intervention.

Kynurenine production and catabolism in fetal sheep with embolized or nonembolized placentas

OBJECTIVE

The effect of maternal tryptophan loading on fetal plasma and brain, kynurenic acid, and quinolinic acid concentrations was compared in late gestation fetal sheep with either chronically embolized or nonembolized placentas.

STUDY DESIGN

The placentas of 4 ewes were embolized by daily injection of mucopolysaccharide microspheres into the umbilical artery from 120 days gestation in amounts sufficient to reduce the fetal arterial PO2 to < or = 12 mm Hg. Four fetuses with nonembolized placentas were the control group. At 135 to 138 days gestation, the ewe received an infusion of tryptophan (100 mg/kg, intravenously) or an equivalent volume of saline solution (100 mL) over 2 hours. Maternal and fetal arterial blood samples were obtained between 2 and 48 hours from the start of the infusion for the measurement of plasma tryptophan and kynurenine metabolites. Brains were then obtained from embolized and nonembolized fetuses 24 hours after a further maternal tryptophan loading experiment and from nonembolized non-tryptophan-treated fetuses for analysis of regional kynurenic acid and quinolinic acid content.

RESULTS

Maternal tryptophan infusion resulted in a significant increase of kynurenine in fetal plasma, but this increase was significantly smaller in fetuses with an embolized placenta compared with a nonembolized placenta. Both kynurenic acid and quinolinic acid levels increased significantly in fetal plasma, with no differences between the groups. Kynurenic acid and quinolinic acid levels were increased in all regions of the fetal brain after maternal tryptophan loading, but these increases were greater in the fetuses with an embolized placenta, compared with a nonembolized placenta.

CONCLUSION

Fetal tryptophan and kynurenine metabolism is significantly altered when placental function is chronically compromised in late gestation. The decreased production of kynurenine from tryptophan may result from the compromise of hepatic function in the fetus, whereas the increased production of kynurenic acid and quinolinic acid in the brain is likely to reflect alterations of metabolism of tryptophan and kynurenine to these neuroactive products by glial cells in the fetal brain.

Regional changes in kynurenic acid, quinolinic acid, and glial fibrillary acidic protein concentrations in the fetal sheep brain after experimentally induced placental insufficiency

OBJECTIVE

This study was undertaken to examine the effects of chronic embolization of the umbilical circulation during late gestation on regional concentrations of quinolinic acid and kynurenic acid (neuroactive products of tryptophan catabolism) and of the astrocyte-associated glial fibrillary acidic protein in the fetal brain.

STUDY DESIGN

Pregnant ewes bearing fetuses with long-term catheter placement were treated daily with injections of either saline solution (n = 4; control group) or mucopolysaccharide microspheres (n = 5; embolized group) into the umbilical circulation through a femoral artery catheter between 120 and 140 days' gestation. The fetuses in the embolized group received sufficient microspheres each day to reduce and maintain the femoral arterial PO2 at < or =12 mm Hg. Autopsies were performed at 140 days' gestation to obtain the fetal brain for chemical analysis.

RESULTS

Umbilical embolization resulted in nonacidemic hypoxia and hypoglycemia at 140 days' gestation. Quinolinic acid concentrations in the embolized group were significantly increased in the medulla, pons, midbrain, hypothalamus, and hippocampus, whereas kynurenic acid concentrations in the embolized group were reduced in the hippocampus and hypothalamus. There were significant reductions in glial fibrillary acidic protein contents in the occipitoparietal cortex, hippocampus, and pons in the embolized group.

CONCLUSION

Placental compromise during late pregnancy had effects on kynurenine metabolism and astrocyte function in some regions of the fetal sheep brain. We suggest that these changes increase the vulnerability of the brain to asphyxial injury during late gestation and the perinatal period.

Effect of hypoxia on respiratory activity in the foetus

1. Breathing movements stop soon after the induction of hypoxia in foetal animals, a response attributed to the active inhibition of the respiratory centres. Separate studies using punctate lesions have identified lateral pontine and thalamic sites in foetal sheep from which respiratory inhibition may arise, but whether either of these loci are actually oxygen sensitive or whether they receive input from other regions responsive to hypoxia, has not been established. 2. FOS immunocytochemistry was used to identify neuronal pools activated by hypoxia in the brain of late-gestation foetal and newborn sheep. FOS-positive cells were found in the pons in regions corresponding to the medial parabrachial and Kolliker-Fuse nuclei and were shown to be catecholaminergic. Neurons in this pontine region were also activated by the piperizine drug almitrine, which, like hypoxia, inhibits respiratory output in the foetus but is a respiratory stimulant in the newborn and adult. Because these pontine neurons do not express FOS protein after challenge with hypoxia or almitrine in the newborn lamb, we argue that they are crucial to the hypoxic inhibition of respiratory activity in foetal life. 3. The role of the placenta in determining these foetal responses and how this may change at birth is discussed.

Tryptophan metabolism in pregnant sheep: increased fetal kynurenine production in response to maternal tryptophan loading

OBJECTIVE

The effects of a tryptophan load on the plasma concentration of kynurenine, the precursor for the production in the brain of the neuroactive products kynurenic acid and quinolinic acid, were determined in pregnant sheep at midgestation and late gestation and in nonpregnant sheep.

STUDY DESIGN

Pregnant ewes were given an intravenous infusion of 100 mg/kg L-tryptophan during 2 hours at 95 to 98 days' gestation (n = 4) or 135 to 138 days' gestation (n = 10). Nonpregnant ewes (n = 6) were studied in late estrus. Arterial blood samples taken from 2 hours before to 48 hours after the start of the infusion were used for analysis of plasma tryptophan, kynurenine, and cortisol concentrations.

RESULTS

Tryptophan loading at both gestational ages resulted in significantly greater increases in kynurenine concentrations in fetal plasma (at 95-98 days' gestation, from 5.7 +/- 1.2 micromol/L [baseline] to 247.9 +/- 86.7 micromol/L (peak); at 135-138 days' gestation, from 9.0 +/- 2.3 micromol/L [baseline] to 289.0 +/- 194.0 micromol/L [peak]) than in maternal plasma [at 95-98 days' gestation, from 4.6 +/- 0.8 micromol/L [baseline] to 118.0 +/- 79.7 micromol/L [peak]; at 135-138 days' gestation, from 4.8 +/- 2.9 micromol/L [baseline] to 98.3 +/- 67.8 micromol/L [peak]). It took longer for kynurenine concentrations to return to basal values in the fetus (24-30 hours) than in the ewe (8-12 hours). The kynurenine responses in pregnant and nonpregnant ewes were not different from each other.

CONCLUSION

The production of kynurenine from tryptophan is significantly greater in the fetal lamb than in the pregnant or nonpregnant adult ewe.

Kynurenic acid in brain and cerebrospinal fluid of fetal, newborn, and adult sheep and effects of placental embolization

Concentrations of the endogenous glutamate receptor antagonist kynurenic acid (KA) were measured in various brain regions and in cisternal cerebrospinal fluid of fetal, newborn, and adult sheep. KA concentrations were significantly higher in the fetal brain and cerebrospinal fluid at 90 and 140 d gestation compared with postnatal ages. In fetuses of 132-139 d gestation, KA concentrations in cerebrospinal fluid collected by drainage from an indwelling cisternal catheter increased significantly after infusion of the organic acid transport inhibitor probenecid (100 or 200 mg/kg, i.v.) indicating active transport of KA out of the fetal brain. In fetuses in which the umbilical circulation had been chronically restricted from 120 to 140 d gestation by partial embolization of the placenta, plasma concentrations of the KA precursor kynurenine were significantly lower than in control fetuses, and KA concentrations in the hypothalamus and hippocampus were significantly reduced; other brain regions were not affected. These results indicate that the production of KA is higher in the fetal brain compared with the newborn and adult brain. Because KA diminishes the risk of excitotoxic neuronal damage under hypoxic-ischemic conditions, the high levels of KA in the brain before birth may have a neuroprotective function. The decrease of KA concentrations in the hypothalamus and hippocampus after umbilical embolization suggests that, after chronic hypoxia in utero, these regions of the brain may become more vulnerable to subsequent episodes of acute hypoxia or ischemia encountered in late gestation or during parturition.

Characterization of pontine neurons which respond to hypoxia in fetal sheep

Hypoxia causes apnea and postural muscle hypotonia in fetal sheep, a response thought to arise by descending inhibition from a group of lateral pontine neurons that express FOS protein after hypoxia. To determine the neurochemical phenotype, and whether these neurons project to the cervical spinal cord, the retrograde tracer CTB-gold was injected into the C5-C8 ventral horn of four fetal sheep at 110 days gestation. Then, at 135 days each fetus was made hypoxic for 2 h by allowing the mother to breathe 7-8% O2. Immunocytochemistry showed that FOS-positive neurons in the subcoeruleus and Kolliker-Fuse regions of the pons were catecholaminergic, but not cholinergic or GABAergic, and a proportion of them contained CTB-gold particles, indicating direct connection with the cervical spinal cord. We suggest that these pontine neurons inhibit respiratory and postural muscle activities during hypoxia in fetal sheep.

The distribution of FOS-immunoreactive neurons in the brainstem, midbrain and diencephalon of fetal sheep in response to acute hypoxia in mid and late gestation

FOS immunohistochemistry was used to map the distribution of neuronal pathways activated by hypoxia in fetal sheep. Conscious pregnant sheep were exposed to hypoxia (7-9% O2, 1-2% CO2, balance N2) for 2 h at either 100-105 days (n=5) or 130-133 days gestation (n=5); term is approximately 147 days. The hypoxia caused cessation of breathing movements at both fetal ages, and increased FOS staining in the medulla (area postrema, dorsal motor nucleus of vagus, nucleus solitary tract, ventrolateral medulla); pons (locus coeruleus and subcoeruleus, lateral and medial parabrachial nuclei); midbrain (habenula, periaqueductal grey, substantia nigra, areas ventrolateral to Red Nucleus); and hypothalamus (anterior and lateral hypothalamic areas, paraventricular and supraoptic nuclei). The results were essentially the same at both gestational ages, except that hypoxia increased FOS-staining in the habenula only in the older fetuses. The presence of FOS protein in pontomedullary cardiorespiratory nuclei at 100-105 days gestation indicates that the peripheral chemoreceptors respond to hypoxia at this early age, and in the subcoeruleus and medial parabrachial regions of the pons is consistent with lesion studies suggesting these areas mediate the inhibition of fetal breathing in response to hypoxia. FOS staining in the ventrolateral periaqueductal grey and habenula was unexpected, and suggests that pathways normally involved in response to noxious stimuli, or which are part of the hypothalamic 'defense' response are activated by hypoxia in the fetus. Some FOS-labelling could arise secondarily as a consequence of the cardiovascular and endocrine responses to hypoxia.

Effects of the respiratory stimulant almitrine on breathing and FOS expression in the brain of fetal and newborn sheep

Almitrine is a piperazine derivative known to stimulate breathing in the adult but cause apnea in fetal sheep. In fetal sheep (127-133 d gestation; term = 147 d) we confirmed this finding, but found that almitrine (4 mg/kg, i.v. or intra-arterial) had a biphasic effect, briefly stimulating and then suppressing breathing movements for at least 3 h. In 2- to 3-d-old (n = 4) and 7- to 14-d-old (n = 4) lambs almitrine increased both tidal volume and breath frequency, increased arterial partial pressure of oxygen and pH, and decreased partial pressure of carbon dioxide. The changes of tidal volume, partial pressure of oxygen and partial pressure of carbon dioxide were less in the 2- to 3-d-old compared with the 7- to 14-d-old lambs. The distribution of the nuclear phosphoprotein FOS, a marker of neuronal activation was examined in fetal and newborn brains. FOS protein was increased in cardiorespiratory areas of the medulla and pons, in the periaqueductal region of the midbrain, and in the supraoptic and paraventricular regions of the hypothalamus. In the pons, FOS protein was increased in the medial parabrachial and subcoeruleus nuclei in the fetuses but not in the 2- to 3- or 7- to 14-d-old lambs. These observations are similar to those reported for hypoxia, and consistent with the hypothesis that both almitrine and hypoxia inhibit fetal breathing movements by an action on a select group of pontine neurons. Whether these neurons respond directly to these stimuli or receive input from the other centers is yet to be elucidated. The mechanisms that change the almitrine (and hypoxia) response from inhibition to excitation at birth have not been identified, but may be important in preventing apnea in the newborn.

Prenatal development of somatosensory primary afferent connections in the sheep

A summary is presented of recently published studies on the structural and functional development of cutaneous and muscle receptors and the connections of their afferent fibres in fetal sheep (n = 26) aged between 67 and 143 days gestation (term, 146 days). In these studies it was shown that primary afferent fibres projected to, and made synaptic connections with, dorsal horn neurons in lumbosacral spinal cord by 56-61 days gestation. Sensory innervation of the skin occurred later by about 75 days gestation and, at this age, stimulation of the skin first activated cutaneous afferent fibres and evoked a discharge in dorsal root ganglion and dorsal horn neurons. Muscle stretch first activated muscle spindles and evoked a discharge in dorsal root ganglion cells by about 75 days. Prior to this (by about 67 days) primary afferent fibres had begun to innervate motoneuron pools in the spinal cord, and motor nerves had begun to innervate muscle fibres. Both muscle spindle and cutaneous innervation were relatively simple at mid gestation indicating that the structure of sensory receptors need not be complex in order to generate a response. Neural pathways necessary for reflex activity involving muscle spindles are therefore present and functional by mid gestation as are cutaneous pathways projecting from the skin to the spinal cord.

The development of cutaneous afferent pathways in fetal sheep: a structural and functional study

In this study we have examined the functional and structural development of cutaneous afferent pathways in the fetal sheep hindlimb from 67 to 143 days of gestation. The earliest age at which extracellular discharges could be evoked in dorsal root ganglia and in dorsal horn cells by natural cutaneous stimulation was 75 days. The majority of cells responded to light stroking or indentation of the skin (low threshold) although some cells responded to intense squeezing (high threshold). With increasing gestational age, the majority of cells continued to respond to low threshold stimuli with cells responding to intense mechanical stimuli being recorded less frequently. Dorsal root ganglion cells responding to the cutaneous application of noxious heat (> 45 degrees C) were first observed at 107 days. We have also shown that fibres projecting from dorsal horn cells and presumably entering the spinothalamic tract carry signals at least as far as the mid thoracic spinal cord by 104 days. Ultrastructural studies of the skin revealed bundles of unmyelinated axons located to blood vessels throughout the dermis at 68 days. Innervation of the skin was first observed by silver staining at 75 days when fibres could be seen running along blood vessels in the dermis. With increasing gestational age fibres were seen traversing the dermis to innervate the epidermis as free nerve endings. Wool/hair follicle innervation was first evident at about 100-106 days with only a few follicles being innervated at this age. By 115 days, nerve endings had begun to form circumferential wrappings around follicles and by 134 days lanceolate terminals were observed around the base of the follicles. Biocytin-labelled afferent fibres entered the dorsal horn at 56 days. This initial innervation was sparse but by 76 days there was a marked increase in both the number of afferent fibres entering the dorsal horn and in the extent of their arborisation. Ultrastructural studies revealed that terminals immunoreactive for calcitonin gene-related peptide were present in lamina I as early as 61 days. The period of maximal synaptogenesis and synaptic maturation of this group of terminals appeared to occur between 87-128 days. Therefore the commencement of activation of dorsal horn cells by natural cutaneous stimulation occurs by mid gestation (75 days) in the fetal sheep. This is at the same time or just after peripheral nerves first innervate the skin and about 2 weeks after primary afferent terminals can be identified in the dorsal horn.

The ontogeny of [125I]rat-alpha-CGRP binding sites in the spinal cord of sheep: a prenatal and postnatal study

In this study we describe the ontogeny of [125I]rat-alpha-calcitonin gene-related peptide binding sites in the spinal cord of fetal and postnatal sheep. The density and distribution of binding sites has been compared with the localization of calcitonin gene-related peptide like-immunoreactivity at corresponding stages of development [Nitsos I. and Rees S. (1993) Neuroscience 54, 239-252]. At 68 days of gestation (term = 146 days), the earliest fetal tissue examined, there was no evidence of binding sites in lamina I or the outer regions of lamina II (lamina IIo), although there was a sparse distribution of binding sites in the inner region of lamina II (lamina IIi). By comparison, binding appeared to be more marked in laminae III-V and more concentrated again in laminae VI-X. This distribution essentially remained constant until 124 days, when there appeared to be a marked increase in the density of binding sites throughout the gray matter, particularly in the dorsal horn in the lateral extent of both lamina IIo and IIi as well as in laminae III, V and VI. This increase was also observed in the intermediate zone (lamina VII) and in lamina X. Binding in the ventral horn, which was diffuse until this stage, now became particularly dense in the medial and lateral regions of the horn. From 124 days to one month postnatal, there was no marked change in the density or distribution of binding sites.(ABSTRACT TRUNCATED AT 250 WORDS)

The structural and functional development of muscle spindles and their connections in fetal sheep

In this paper we have studied the structural and functional development of hindlimb muscle receptors and the connections of their afferent fibres in fetal sheep (n = 26) from 67-143 days of gestation (term = 146 days). By recording extracellular discharges in dorsal root ganglia (L7, S1) we have shown that muscle spindle afferents first respond to a ramp-and-hold stretch at mid-gestation (approximately 75 days). Silver-stained preparations of muscle spindles revealed that afferent fibres are just beginning to form annulospiral windings at this age. It therefore appears that the annulospiral formation is not a necessary requirement for the generation of the response. By 87-92 days some receptors had developed a discharge at resting muscle length. Discharges were generally more robust and easier to elicit and static and dynamic components could be identified in the response to stretch. Although static sensitivity was generally low it was more evident than dynamic sensitivity. By 107-115 days it was possible to clearly distinguish between muscle and tendon afferents and to tentatively classify muscle responses as originating from primary or secondary afferent spindle endings. With increasing gestational age there was a progressive increase in the length and complexity of the spindle innervation in parallel with the maturation of functional activity. Biocytin injections into the dorsal root ganglia revealed afferent projections to the motoneuron pools by 67 days. Silver-staining of muscles showed that innervation of extrafusal fibres was also present by this age. We therefore conclude that the neural pathways necessary for reflex activity involving muscle spindles are present and functional from early in gestation and could contribute to early fetal movements.

Development of immunoreactivity for calcitonin gene-related peptide, substance P and glutamate in primary sensory neurons, and for serotonin in the spinal cord of fetal sheep

In this study we have described the ontogeny of immunoreactivity for calcitonin gene-related peptide, substance P and glutamate in primary sensory neurons, and for serotonin in the sacral spin cord, of fetal sheep (n = 37) from 56 to 140 days of gestation (term = 146 days). A few fine, varicose fibres immunoreactive for calcitonin gene-related peptide were present in Lissauer's tract, the dorsolateral funiculus and in laminae I and V in the dorsal horn of the spinal cord at 56-61 days of gestation. At this age, two groups of intensely staining immunoreactive cells were present in the motoneuron pool in laminae VIII and IX in the ventral horn of the spinal cord. By 77 days, immunoreactive fibres were also present in laminae II and X. With advancing gestational age, an increase in the intensity of staining was observed throughout the cord to term, with the exception of laminae VIII and IX, where a decrease was seen. Intense staining of cells in the motoneuron pool was evident until c. 128 days, after which time staining became very faint. Fine fibers immunoreactive for substance P were present in Lissauer's tract and lamina I of the spinal cord at 56-61 days of gestation. They were also present throughout laminae IV-VI and X as well as throughout the entire ventral horn. Immunoreactive fibres in lamina II were evident by 77 days. The staining increased in density but remained similar in distribution with increasing gestational age to term in the dorsal horn, but decreased markedly in the ventral horn. Cells immunoreactive for substance P were evident from 56 days, particularly on the border of laminae II and III, until late in gestation. Ultrastructural studies showed that axon terminals immunoreactive for calcitonin gene-related peptide and for substance P were present in lamina I by 61 days. Immunoreactivity for glutamate was evident at 83 days in dorsal root fibers and also in lamina I and II, where it was more prominent in cells than in fibres. At all ages examined, the dorsal horn stained more intensely than the ventral horn. Immunoreactivity for glutamate and neuropeptides appeared in the cells and fibres of dorsal root ganglia at 97-100 days. In the skin, immunoreactivity for calcitonin gene-related peptide and substance P was present at 85 days, some time after its appearance in the cord. Fibres immunoreactive for serotonin appeared in lamina I, at the neck of the dorsal horn and in the ventral horn at 83 days of gestation.(ABSTRACT TRUNCATED AT 400 WORDS)

Prenatal development of cutaneous afferent connections in the spinal cord of fetal sheep. A physiological and neurochemical study

In this study we have examined the physiological and neurochemical development of the cutaneous afferent pathways from the hindlimb to the spinal cord in fetal sheep. We have shown that somatosensory input from the hindlimb evokes activity in DRG neurons at 87d gestation and in cells in the dorsal horn at 92d (term, 146d). There is evidence of immunoreactivity for substance P, calcitonin gene-related peptide and glutamine several days prior to this at 77-80 days. The implication of these findings are discussed.

The effects of intrauterine growth retardation on the development of neuroglia in fetal guinea pigs. An immunohistochemical and an ultrastructural study

The effects of intrauterine growth retardation on the development of myelinating oligodendrocytes and astrocytes in the brain and spinal cord of the fetal guinea pig have been examined using immunohistochemical and ultrastructural techniques. As judged by immunoreactivity for myelin basic protein, the extent of myelination in the spinal cord, cerebral cortex, corpus cellosum and cerebellum was reduced in the growth-retarded fetuses compared with controls at both 52 (n = 4) and 62 days (n = 5) of gestation. As assessed by immunoreactivity for glial fibrillary acidic protein, there were no marked differences between control and growth-retarded brains in the extent or distribution of radial glial cells or astrocytes at 52 or 62 days in the cerebellum. However, in the cerebral cortex at 62 days there was a striking proliferation of astrocytes surrounding cortical blood vessels in growth-retarded fetuses. Ultrastructural studies showed that at 52 days, myelination of the corticospinal tract had begun in the control but was virtually absent in growth-retarded fetuses. At 62 days, the total number of myelinated fibres in growth-retarded fetuses was significantly reduced by 56% (P less than 0.01) compared with control fetuses; however, there was no difference between the groups in the total number of fibres in the corticospinal tract. Where fibres were myelinated the myelin sheath was disproportionately reduced relative to axon diameter. Thus, in intrauterine growth retardation there is a delay in the initiation and in the extent of myelination. This could be due to a reduction in the number of myelinating glia formed and the restricted capacity of those which do form to generate myelin.