Effects And Time Choice Of Enriched Environment On Learning And Memory Ability Of Neonatal Rats With Hypoxic-ischemic Brain Damage

Background and objective:It is widely accepted that Hypoxic-ischemic brain damage (HIBD) could severely impairs neonates' lives and usually results in eternal dysfunction.It is still a disease without any ideal or effective therapy by now.Studies on animals indicated that enriched environmental therapy could increase neuronal plasticity,improve brain function and promote the recovery of brain damage.However,the effects and mechanisms of enriched environment on developmental brain damage remain not fully clarified.It is still a disputation on choosing an optimal opportunity of environmental stimulation therapy. The present study was designed to observe the effects of enriched environment on learning and memory ability and neuronal plasticity of neonatal rats with HIBD,explore the possible neuronal mechanisms and identify the sensitive period of developmental brain receiving environmental stimulation.The purpose of this study was to provide theoretical basis and instruction to taking enriched environmental therapy as a clinical treatment of neonates with HIBD in time.Methods:The Sprague-Dawley(SD) rats' models of HIBD were established by the method of Rice.Then the rats with HIBD were divided randomly into 3 groups:early intervention group(EI),late intervention group(LI) and non-intervention group(NI).The sham- operation rats served as sham-operation group(Sham).Enriched environment intervention was administrated to the EI group rats for 20 day since the 2nd day after HIBD. The same intervention to the LI group rats since the 23rd after HIBD.On the age of 100 day, Morris water maze was used to evaluate the learning and memory ability.Then HE stain and nissle stain were employed to observe the morphological changes and the number of neurons in hippocampus of rats.The immunohistochemical stain was used to detect the gray values of p38 and MAP-2 in hippocampus respectively.Results:1.The water maze scores of EI group in place navigation test and spatial probe test were much higher than that of LI group and NI group(p＜0.01),there was no significant difference compared with Sham group(p＞0.05),and the scores of LI group were higher than that of NI group(p＜0.05).2.The results of HE stain and Nissle stain displayed the morphology in left-sided hippocampus of Sham group was normal,and that of EI group was minimal abnormal,the neurons almost arranged orderly and remained normal,some neurons lost.While the morphology of LI group and NI group were obviously abnormal,the neurons arranged disorderly,many neurons lost and the nucleuses were not clear.The nissl stain showed that the number of left-sided hippocampal neurons of EI group was much more than that of LI group and NI group(p＜0.01),but less than that of Sham group(p＜0.05).The number of LI group was much more than that of NI group(p＜0.05).3.The transmission electron microscope displayed that the synaptic ultrastructure of hippocampus CA1 pyramidal neuron of EI group was almost normal compared with Sham group.While the synaptic ultrastructure of hippocampus CA1 pyramidal neuron of LI group and NI group were distorted obviously,the pre-synaptic vesicles were decreased,the synaptic cleft became widen,and the post-synaptic densities were attenuated;4.Immunohistochemical analysis showed that the expression of p38 and MAP-2 in left-sided hippocampus of EI group significantly increased as compared with that of LI group(p＜0.01).There was no significant difference between EI group and Sham group (p＞0.05),and the expression of p38 and MAP-2 of LI group were stronger than that of NI group(p＜0.05).Conclusions:1.EE intervention could decreased the pathological damages in left-sided hippocampal after hypoxic ischemia and promoted the ability of learning and memory of rats with HIBD,which indicated that EE was good for recovery from brain damage.2.The effects of Early intervention on rats whith HIBD was better than that of late intervention,which indicated that brain developmental critical period is the best occasion of enriched environment on the brain function rehabilitation of hypoxia-ischemic brain damage in neonatal rats.3.The neuronal plasticity of EI group was promoted more than that of LI group.The changes of p38 and MAP-2 in their hippocampuses were possibly associated with the influence of environment on neuronal plasticity at different developmental stage of HIBD.