Pathophysiological consequences of an in utero hypoxic insult on the foetal rat brain

The occurrence of one or more hypoxic-ischaemic episodes during the perinatal period is believed to contribute to a range of non-progressive neuronal deficits in the human neonate. There are presently no widely accepted animal models of this clinical condition that allow the study of the pathophysiology of perinatal hypoxia. The aim of this study was to investigate the effects of an in ute.ro hypoxic insult on the near-term foetal rat brain. An hypoxic insult of 10, 20 or 30 minutes was created by clamping the utero-placental vessels of one uterine horn in an anaesthetised pregnant Lister hooded rat at 22 days gestation. After completion of the occlusion period the dam was killed and the foetal rats were delivered by Caesarian section and resuscitated. Alternatively the clamps were removed, the uterine horn placed back inside the dam and the wound sutured. After a recovery period of 20 minutes, during which time the dam was not anaesthetised, the dam was killed and the foetal rats were delivered by Caesarian section and resuscitated. Biochemical analysis of homogenised and deproteinised brain tissue samples from the neonatal rats showed a level of metabolic deficit typically associated with irreversible neuronal necrosis in adult models of hypoxic-ischaemia. The level of brain tissue lactate in rats subjected to a 30 minute clamp without recovery was 19.77 ± 0.8nmols/mg brain tissue whereas in control rats the level of lactate was 7.98 + 0.2nmols/mg brain tissue (p < 0.001). Brain tissue ATP declined by more than 80% from 2.67 + 0.2nmols/mg brain tissue in control rats to 0.59 ± O.lnmols/mg brain tissue in hypoxic rats (p < 0.001). Other metabolites also showed changes consistent with a major hypoxic insult. Hypoxic neonatal rats and their littermate controls were cross-fostered onto nursing Cob-Wistar dams to enable measurements during the postnatal period. Assessment of the increase in weight and head-to-tail length of hypoxic and control rats from pnd0-pnd42 showed no significant difference between the two groups. Activity ratings and development of the righting reflex were also similar for the two groups. Despite the metabolic deficit indicated by the biochemical data histological analysis showed a lack of gross lesions or widespread neuronal necrosis, although there was some indication of an increase in the number of dead or dying neurons around the CA3 region of the hippocampus and the hind brain nuclei. The brains of adult rats that had been subjected to an in utero hypoxic insult before birth exhibited no gross neuropathological changes. However, analysis of the hippocampal CA1 width of the adult rat brains showed that hypoxic CA1 regions were significantly narrower (3-6 %, p < 0.05) than control CA1 regions. This reduction might reflect changes in hippocampal neuronal density, cell size or alterations in dendritic outgrowth. A place navigation task was used to assess spatial memory in the rats at 3 months of age. The rats were trained to find a hidden platform in a 2m diameter water maze made opaque by the addition of powdered milk. Acquisition of the task was achieved at a similar rate by both the control and the hypoxic rats. In contrast, the performance of rats with hippocampal lesions induced by injection of ibotenic acid was significantly impaired (p < 0.001). In a transfer test control rats spent 56.59 ± 3.8% of the testing period in the training quadrant whereas hypoxic rats spent 51.13 + 4.7% of the testing period in the training quadrant (p > 0.05). Hippocampally lesioned rats spent only 31.28 ± 6.9% of the testing period in the training quadrant which was significantly less than both the control and the hypoxic rats (p < 0.001). This study has shown that despite a significant metabolic deficit those rats which survive an in ute.ro hypoxic insult suffer only limited and discrete neuropathological changes and no apparent long-term physiological or cognitive deficits. These results suggest that the nearterm foetal rat brain has a high level of resistance to the long term consequences of a severe hypoxic insult.