Effects Of Nitric Oxide Synthase Inhibitor L-NAME On Cognitive Behaviour And The Expression Of INOS In Hippocampus Of Chronic Cerebral Ischemia Rats

In recent years, chronic cerebral ischemia being already confirmed is a common pathological mechanism of various diseases such as vascular dementia , Alzheimer's disease. At present, we usally adopt the permanent occlusion of bilateral common carotid arteries(2VO) to produce the chronic cerebral ischemia models at home and abroad, and already being confirmed commonly, after ligation of bilateral common carotid arteries, cerebral blood flow comes down obviously, but since the rat brain basilar artery rings is developed, compensatory finely, the blood flow in any region of rat brain can not being complete terminated, therefore the rat brain is in one kind of hypoperfusing state all the time, and chronic hypoperfusion initiates a series of histopathology, hemodynamics and behavior consequences. Recent studies have reported that, the longer the ischemic time, the worse the cerebral ischemia-induced neurological damages. Nitric oxide has being shown to play an important role in the pathogenesis of neuronal injury during cerebral ischemia. It may influence pathophysiology of brain ischemia in a complex way depending on the sources of its production. Appropriate level of NO is beneficial to the regulation of brain blood flow, excessive NO release has being shown to be cytotoxic, can aggravate neurological damages. Followed cerebral ischemia, iNOS is transcriptionally induced primarily through the actions of cytokines and its activity continues for many days after induction. The resultant output of NO is sufficiently high to allow significant accumulation of the oxidative product nitrite, and participate in delayed neurological damages. A study has proved that iNOS activity rises after cerebral ischemia/reperfusion in rats, especially in the region of hippocampus. It indicates that iNOS plays an important role in the process of brain injury, but if iNOS continue to influence the process of chronic cerebral ischemia, how to express, and which cell sources of its production, still need to be study further.In the experiment, permanent ligation of bilateral common carotid arteries (2VO) was used to produce the chronic cerebral ischemia models. To determine the ability of the learning and memory after the Morris water maze task training and the expression of iNOS in hippocampal CA1 area of rats after 2VO, administration of the nitric oxide synthase (NOS) inhibitor Nω-nitro-L-arginine methylester(L-NAME, 20mg/Kg/d in drinking water) was used in an intervention study. Males Wistar rats weighing 250～300g were randomly divided into three groups: the sham-operation group, experimental group, and L-NAME group. Every group were divided further into three subgroups: 2moth, 4month, 6month (ischemia for 2 months, 4 months and 6 months). Morris water maze experiment was adopted to test the rats'spatial learning and memory each group. The coronal slides of cerebral tissue were stained by HE to explore histopathological changes of experimental group and L-NAME group at 2months, 4 months and 6months, and by immunohistochemical technique, to identify the expression of iNOS in hippocampus CA1 area. The positive cells were counted for statistic analysis.The result showed: Pathological changes: The degeneration, necrosis and detachment of neuron in hippocampus could be seen in the rats of experimental group and L-NAME group, the longer the ischemic time, the worse the neurological damages. In 6moth experimental group, we observed serious lose of neurons. Compared with experimental group, L-NAME group changed less significantly. In the Morris water maze task, rats with common carotid ligation exhibited a learning and memory deficit, but the impairment was less severe in L-NAME group. Immunohistochemistry: We could not find any iNOS positive cell of hippocampal CA1 area in sham-operation group, reversely, the expressional level of iNOS increased gradually from 2moth to 6moth experimental group, on the other hand, the expression level of iNOS was reduced in L-NAME group.Several lines of evidence suggested that the increased expression of iNOS in hippocampal CA1 area and NO played an neurotoxic role in the process of brain injury. Followed chronic cerebral ischemia, inflammatory cytokins were produced, which then induced over expression of iNOS. Increased levels and activation of iNOS appear to contribute to a somewhat delayed neurotoxicity following ischemic injury. From the whole experiment we can conclude:1. Chronic cerebral ischemia induced by permanent internal carotid ligation causes pathological damages to the brain tissues, leads to the degeneration, detachment and loss of neuron, L-NAME may lighten neurological damages.2. Chronic cerebral ischemia impairs the ability of rats'spatial learning and memory, L-NAME can ameliorate the impairment.3. The expressional level of iNOS in hippocampal CA1 area increased gradually following chronic cerebral ischemia, L-NAME can depress the increasing. 4. The effect that L-NAME depresses the increasing of the expressional level of iNOS in hippocampal CA1 area, further restrains cytotoxic NO release under pathological conditions, may be a protective mechanism from chronic cerebral ischemia-induced neurological damages.