Hypoxic-ischemic brain damage (HIBD) occurs at a rate of 3–5/1,000 full-term live births in developed countries and ≤10-fold higher in developing countries (1). Following hypoxia-ischemia (HI), ~45% of newborns succumb or have permanent cognitive impairments, potentially including cerebral palsy, and there are currently no effective therapies (2).

Hyperbaric oxygen (HBO) therapy has been identified as a clinical therapy for HIBD for a number of years. Animal experiments and clinical trials have shown that HBO performed within 6 h after HIBD may achieve favorable outcomes and promote the long-term neurological recovery (3–6). In addition, numerous studies in China have demonstrated that HBO can reduce the disability and mortality of HIBD (7). However, there are concerns regarding the safety and validity of HBO treatment due to oxygen toxicity, which may increase the infarct area (3). A previous study demonstrated an increase in lipid and protein oxidation products in the lung tissues of rats accompanied by increased antioxidant enzyme activities when HBO was continued for >20 sessions (8). In addition to the lung being the entering site of hyperoxic injuries, CNS is mainly accepted as another important target for oxygen exposure in toxic amounts (9). However, whether HBO will exacerbate lipid peroxidation causing secondary injury to the brain or lead to neonatal pulmonary and retinal hypoplasia (10) remains to be elucidated. The aim of the present study was to detect the effect of HBO on superoxide dismutase (SOD) and malondialdehyde (MDA) from brain homogenate and flash visual evoked potential (F-VEP) in newborn rats with HIBD through the establishment of the HIBD newborn rat model, to explore the influence of HBO on brain lipid peroxidation and visual development, and to provide experimental evidence for the clinical application of HBO.

Compared with the hypoxic ischemia model caused by ligating the side carotid artery postnatally, intrauterine HIBD animal models better replicate the common syndrome of fetal distress, which is more similar to clinical conditions. The intrauterine HIBD animal models were established by ligating the side cornual artery and delayed cesarean section (11). The neonatal rats in the control group came from the other side of the uterus where the cornual artery was not ligated. The rats of the experimental group and the control group were from the same mother, which may reduce any genetic influence. The rats had moderate or severe brain damage in HIBD and may have had sequelae such as mental retardation and cerebral palsy. This is why only rats with moderate or severe brain damage were used as HIBD models in the present study.

HBO was widely adopted as a clinical therapy for anoxia disease as it can increase the blood oxygen content and partial pressure of oxygen in arteries, enhance the utilization of oxygen in plasma and tissue fluids, and improve the diffusion of oxygen through tissue fluids from the capillary to the cells. However, whether the application of HBO therapy in neonates was effective or if it would impede the development of pulmonary and retinal structures in immature individuals remains to be elucidated.

F-VEPs are bioelectrical activity caused by the visual stimulations along different sections of optic and visual pathway, which were used to evaluate neural pathways from the optic chiasm level to part under ventricle to the visual cortex level, and it should be consistent with vulnerable sections of classic HIBD. The P1 waves of F-VEPs are stable composite waves based on positive waves, of which the latency is an important index in F-VEP tests. The P1 latency is associated with myelination of nerve fibers and synaptic transmission efficiency, and is independent from anesthetics or sedatives. Shorter P1 latencies indicate more perfect neurodevelopment and a more complete visual path (17). In the present study, rats in the HIBD group showed longer P1 latencies compared with the control group prior to HBO treatment, which implied decreased nerve fiber myelination and synaptic transmission efficiency in neonatal hypoxic-ischemic brain. After continuous HBO treatment for 14 days (1 h/day), the P1 latencies of the rats at 4 weeks postnatal in the HIBD group were evidently shorter compared to those without HBO therapy, and were similar to those of the rats in the control group. The result suggested that HBO treatment cannot impede normal visual development.

Synapses are specialized structures conveying information between neurons in the nervous system, and synaptic knobs contain numerous vesicles, which are closely associated with neurotransmitter release (18). Postsynaptic densities are the main components of postsynaptic membrane, of which the alteration in thickness is one of the most important morphological bases for normal synaptic function (19). Synaptic space contains a lot of lytic enzyme which can decompose the neurotransmitter. The wider the synaptic space is, the longer the time that neurotransmitter conduction takes; more were degraded during the conduction period, thus causing a reduction of the abilities of nerve conduction and learning and memory. In the present study, transmission electron microscopy showed that there were evident morphological changes in the hippocampal CA3 neurons of rats in the HIBD group, of which the nuclear membranes disappeared in part, the mitochondria cristae were vague, the number of presynaptic vesicles decreased, the postsynaptic density became thin, and the synaptic clefts were unevenly widened; whole synapses were also destroyed. However, in the HBO-treated HIBD group, no significant differences were identified in CA3 synaptic ultrastructures compared with that in the control group under the transmission electron microscope. The latency of F-VEP of rats in HIBD following the HBO intervention was shorter than that in the control group, which suggested that early HBO intervention could maintain a relatively normal synaptic structure and their functions by reducing synaptic ultrastructural damage caused by hypoxia ischemia and promoting restoration from injuries.

Lipid peroxidation is considered an important reason for immature brain injury, pulmonary hypoplasia and visual damage in oxygen therapy (3,20,21). MDA is an end product in lipid peroxidation which can reflect the extent of lipid peroxidation. SOD is a main antioxidant enzyme in organisms that has an important role in removing extra free radicals. The present results showed that the SOD content of rats in the HIBD group was markedly lower while the MDA content was significantly higher than those in the control group, which implied that HI could reduce the antioxidant abilities and promote lipid peroxidation. Following the HBO intervention, the rats with HIBD showed significantly increased SOD activity and decreased MDA content, which suggested that a HBO intervention could markedly improve the removal of oxygen free radicals and resist lipid peroxidation. The study showed that HBO could enhance antioxidant capacity and decrease the ultrastructural damage induced by hypoxic-ischemia, and facilitate synaptic reconstruction and alleviate injury on immature brain thus promoting the rehabilitation of brain function. These results support that intervention programs and a pressure of HBO (0.02 MPa once a day for 1 h; 14 days total) do not increase oxidative damage of the central nervous system of the HIBD rats. Instead, antioxidant injury is conducive to a protective effect on the brain. Future investigations are required to elaborate upon the mechanism of HBO treatment on the immature brain and to identify the association between the therapeutic effect of HBO treatment and the pressure or time and duration of the HBO intervention.