Prolonged reductions in placental blood flow and cerebral oxygen delivery in preterm fetal sheep exposed to endotoxin: possible factors in white matter injury after acute infection.

OBJECTIVE: Endotoxin causes hypoxemia and white matter injury in the preterm ovine fetus. Because cerebral hypoxia could contribute to brain injury, our objective was to determine the effects of endotoxin on regional cerebral oxygen (O(2)) delivery. To investigate causes of fetal hypoxemia, we also measured placental blood flow. METHODS: We administered endotoxin (lipopolysaccharide, LPS) at 1 microgram/kg (intravenously) to 11 catheterized fetal sheep at approximately 0.7 of term; controls (n = 7) received saline. We measured fetal cerebral blood flow (CBF) and placental blood flow using microspheres, arterial blood gases, arterial pressure, and heart rate. RESULTS: Seven fetuses survived LPS administration (LPS-S) and four died. LPS-S fetuses were hypoxemic at 4-8 hours after LPS. Fetal hemoglobin concentration and hematocrit increased by about 14% at 4 hours after LPS exposure, and mean arterial pressure decreased significantly from 4-8 hours. After LPS, CBF did not change significantly, but total cerebral O(2) delivery decreased by 35.7% at 4 hours and by 28.3% at 8 hours. O(2) delivery to cerebral white matter decreased below pre-LPS values at 4 hours (-35.9%) and 8 hours (-28.6%) after LPS. Relative to pre-LPS values, placental blood flow decreased by 53.3% at 4 hours and 43.0% at 8 hours after LPS. CONCLUSIONS: Immature fetal sheep exposed to LPS had profound reductions in placental blood flow and cerebral O(2) delivery, which could contribute to fetal brain injury. Reduced O(2) delivery to white matter was similar to that in other brain regions. Mechanisms that enable fetal CBF to increase in hypoxemic conditions were apparently ineffective in the presence of LPS.

Extracellular glutamate levels and neuropathology in cerebral white matter following repeated umbilical cord occlusion in the near term fetal sheep.

Umbilical cord occlusion causes fetal hypoxemia which can result in brain injury including damage to cerebral white matter. Excessive glutamate release may be involved in the damage process. This study examined the relation between extracellular glutamate levels in the cerebral white matter of the ovine fetus during and after intermittent umbilical cord occlusion and the degree of resultant fetal brain injury. Fetal sheep underwent surgery for chronic catheterisation and implantation of an intra-cerebral microdialysis probe at 130 days of gestation (term approximately 147 days). Four days after surgery (day 1), seven fetuses were subjected to 5x2 min umbilical cord occlusions, and on the following day (day 2) they were subjected to either 4 or 5x4 min umbilical cord occlusions; seven fetuses served as controls. Microdialysis samples were collected before, during and after the umbilical cord occlusions to determine extracellular glutamate levels in the cerebral white matter. Fetal blood gas status was measured and the fetal electrocorticogram was recorded continuously. During the periods of umbilical cord occlusions on both days 1 and 2, fetal arterial oxygen saturation, arterial partial pressure of oxygen and arterial pH decreased (P<0.05) while arterial partial pressure of carbon dioxide increased (P<0.05). All fetuses showed episodes of isoelectric electrocortical activity during umbilical cord occlusions on both days 1 and 2. In fetuses with patent microdialysis probes there were marked increases of glutamate efflux in the cerebral white matter following umbilical cord occlusion. Fetal brains were removed at autopsy on day 5 and subjected to histological assessment. Brain damage was observed in all fetuses exposed to cord occlusion, particularly in the periventricular white matter, with the most extensive damage occurring in the fetuses with the greatest increases in glutamate levels. We conclude that, in the unanesthetised fetus in utero, glutamatergic processes are associated with umbilical cord occlusion-induced brain damage in the cerebral white matter.

Expression of the p75 neurotrophin receptor by striatal cholinergic neurons following global ischemia in rats is associated with neuronal degeneration.

The induction of the p75 neurotrophin receptor (p75NTR) on striatal cholinergic neurons by global hypoxic-ischemia has been reported to promote neuron survival. We have found, however, while the p75NTR-expressing neurons survive the insult for the first 5 days, subsequently they undergo shrinkage, loss of choline acetyl transferase (ChAT) expression, and more than 96% are eventually lost by 8 days. In contrast ChAT-expressing cells in the surrounding region of the infarction, do not express p75NTR and there is no evidence of neuronal loss. These results suggest the expression of p75NTR on cholinergic interneurons of the rat striatum is associated with delayed neuronal degeneration.

Relation between damage to the placenta and the fetal brain after late-gestation placental embolization and fetal growth restriction in sheep.

OBJECTIVE: Our aim was to determine the effects of 30 days of placental insufficiency on fetal brain development and to relate placental damage to the degree of fetal brain injury. STUDY DESIGN: Umbilicoplacental embolization was induced from 110 to 140 days of gestation (term, 147 days) in 7 fetal sheep, such that fetal arterial oxygen saturation was maintained at 50% of pre-umbilicoplacental embolization values. Six control fetuses were used. At 140 days the fetal brains and placentas were subjected to structural and histochemical analysis. RESULTS: During umbilicoplacental embolization, fetal arterial oxygen saturation, PaO(2), and pH were reduced (P <.05). Thirty days of umbilicoplacental embolization caused a decrease in cross-sectional area of the placentome (P <.05), with 20% of tissue showing damage. All umbilicoplacental embolization fetuses were growth restricted and had brain damage, most prominently in the cerebral white matter. There was no relation between the extent of placental damage and the severity of fetal brain damage. CONCLUSIONS: The absence of a correlation between damage to the placenta and fetal brain is likely to be caused by variations between individuals in (1) the amount of placenta that is required to be functionally damaged to achieve the prescribed level of hypoxemia and (2) the response of the fetal brain to that level of hypoxemia.

Assessment of learning ability and behaviour in low birthweight lambs following intrauterine growth restriction.

The present study used behavioural tasks to assess learning ability and behaviour in postnatal lambs, and to examine the effects of low birthweight (LBW) and age on subsequent performance. It was hypothesized that intrauterine growth restriction (IUGR) and LBW lead to learning and behavioural deficits in the early postnatal period. IUGR and LBW were induced by umbilico-placental embolization from 120 days of gestational age (g.a.) to the onset of labour. Behavioural studies were performed on 6 LBW and 6 control lambs between 2 and 6 weeks after birth. LBW lambs were born at 139+/-1 days g.a. (2.4+/-0.2 kg) and control lambs were born at 149+/-1 days g.a. (4.5+/-0.4 kg). Three tasks were used to assess the learning ability and behaviour of the lambs: a simple maze, an obstacle course, and a T-maze. LBW lambs took longer to complete the simple maze at all ages, and made a greater number of errors at Week 1 of testing compared to control lambs; the total trial duration and number of errors decreased with age for both groups. In the obstacle course, the times taken to complete the first and third trials were used for analysis; a decrease in trial time and the number of errors from Trial 1 to Trial 3 were indications of the lamb's ability to learn how to negotiate the objects within the course. LBW lambs recorded longer trial durations for the first trial at Week 5 of testing, and for the third trial at Week 4. LBW lambs made more errors for the first trial at Week 5 of testing than control lambs. In the T-maze, there was no significant effect of treatment or age. It was concluded that differences between the groups may have been the result of LBW lambs being prematurely born. The value of these tasks in the assessment of learning ability and behaviour in young lambs is discussed.

High-performance tunable filter.

The tuning performance of a liquid-crystal, electro-optically tunable, Lyot filter is described. This filter is widely tunable across the visible region up to a wavelength of approximately 630 nm and has very high performance characteristics that are preserved over the entire tuning range. These include a narrow passband of 0.1 nm, a large-aperture performance of at least 30 mm, a wide field of view of ?8.5 degrees , and the ability to be tuned to an accuracy that is one tenth of the linewidth of the passband. Possible applications for such a filter include remote sensing, multispectral sensing, and laser environmental and underwater monitoring.

Assessment of visual acuity in the supine position.

OBJECTIVE: To determine whether a modified Snellen eye chart could be used to accurately assess visual acuity (VA) in the supine position. METHOD: This was a prospective study involving ED staff volunteers comparing VA on a standard Snellen eye chart with VA on a size-reduced ceiling-mounted modified Snellen eye chart. RESULTS: Fifty-six volunteers participated. VA ranged from 20/10 to 20/200 on both of the charts, but 87% of the volunteers had VA of 20/50 or better. The VA results for the 2 charts were highly correlated; right eye r = 0.931 and left eye r = 0.953. Weighted ks showed substantial agreement for both eyes; kappa = 0.63 and 0.79 for the right and left eyes, respectively. In only 4 of 112 paired measurements did the VA recorded with the ceiling chart differ by > 1 line from that recorded on the Snellen chart. CONCLUSION: There is an excellent correlation between VAs determined in the erect and the supine positions using the standard Snellen eye chart and the modified ceiling-mounted version. Substantial agreement exists between readings using the 2 charts. Although additional testing is warranted in an ophthalmologically diverse patient population, use of this chart for the assessment of VA in the supine ED patient may allow for earlier VA evaluation.

Identification of osmoresponsive neurons in the forebrain of the rat: a Fos study at the ultrastructural level.

The aims of this study are twofold. The first is to describe the ultrastructural morphology of putative osmoreceptors concentrated in the ventral aspect of the lamina terminalis in the rat forebrain. The second is to determine whether or not these neurons lie within an area which lacks a blood-brain barrier, i.e. the organum vasculosum lamina terminalis. The results describe a compact population of neurons in the ventral part of the lamina terminalis which both respond to an osmotic challenge and project directly to the supraoptic nucleus. Injection of horseradish peroxidase into the circulation, as a marker to define areas of the brain without a blood-brain barrier, indicates that these neurons are in the dorsal aspect of the organum vasculosum of the lamina terminalis. An ultrastructural analysis of the neurons in this area, which respond to an osmotic challenge with an elevation of Fos protein, show them to have no specific morphological characteristics which differentiate them from other, non-responsive neurons in the organum vasculosum of the lamina terminalis. However, one possible exception is that osmotically sensitive neurons have a less indented nucleus, suggesting that they are in a more active state than their non-osmotically sensitive neighbours. It is concluded that neurons in this region of the brain are candidate structures for the "receptors" which mediate vasopressin release in response to an osmotic challenge. The response of only a subset of neurons in the organum vasculosum of the lamina terminalis to an osmotic stimulus, despite an apparent morphological homogeneity and the ability of blood borne agents to reach all parts of the structure suggests that osmoresponsiveness is conferred by unique membrane properties or intracellular processing events. The presence of synaptic input to osmoresponsive cells indicates a potential for integration of other inputs at this level.

bcl-2 transgene expression can protect neurons against developmental and induced cell death.

The bcl-2 protooncogene, which protects various cell types from apoptotic cell death, is expressed in the developing and adult nervous system. To explore its role in regulation of neuronal cell death, we generated transgenic mice expressing Bcl-2 under the control of the neuron-specific enolase promoter, which forced expression uniquely in neurons. Sensory neurons isolated from dorsal root ganglia of newborn mice normally require nerve growth factor for their survival in culture, but those from the bcl-2 transgenic mice showed enhanced survival in its absence. Furthermore, apoptotic death of motor neurons after axotomy of the sciatic nerve was inhibited in these mice. The number of neurons in two neuronal populations from the central and peripheral nervous system was increased by 30%, indicating that Bcl-2 expression can protect neurons from cell death during development. The generation of these transgenic mice suggests that Bcl-2 may play an important role in survival of neurons both during development and throughout adult life.

Replication studies in the 16p+ variant.

We report a further case of the 16p+ chromosome studied by replication banding. The extra euchromatic material was shown to be uniformly light staining, indicating that it is late replicating.