The Study On The Neuroprotection Of Leptin Against Aβ1-42-Induced Impairments In Spatial Memory And In Vivo Hippocampal L-LTP In Rats

Objectives:To investigate the effects of chronical administration of leptin on spatial short term working memory in Y maze, the effects of chronical administration of leptin on spatial reference memory in Morris water maze, the possible mechanism underlying the neuroprotective effects of leptin on long term cognitive behaviors and further explore the neuroprotective role of leptin.Methods:Sprague Dawley(SD) rats weighed between 250-300 g were used in the study. Food and water were available ad libitum. After 3 days adaptation, rats were anesthetized with chloral hydrate(0.3g/kg, i.p.) and positioned on a stereotaxic apparatus(RWD, Shenzhen,China). Cut the scalp and drilled a hole on the skull, next, inserted a cannula into the lateral ventricle and fixed with dental cement. The coordinate of the cannula tip is 1.0 mm posterior, 1.5 mm lateral and 4.0 mm depth from bregma. After 5 days of recovery, 20 μg Aβ1-42 was injected through the cannula into the ventricle. 24 hours later, leptin with 1 μg per day was given through cannula for 10 consecutive days. Then spontaneous alternation in Y maze was conducted. 24 hours after spontaneous alternation, Morris water maze test was done in a circular tank. A camera hanging above the tank was used to record swimming traces of rats. The pool was divided into four quadrants and a hidden platform was set 1-2 cm under water in the targeted quadrant. Each rat was placed into the water facing wall of the tank from different quadrant in all trails. The learning ability of rats was examined using hidden platform test from the 1st to the 5th day, allowing rats to swim 4times per training day. Memory test was evaluated by 2 times of probe trail on the 6th day after removing platform out of the pool. Visual and locomotor ability of rats were examined using visual platform test. Escape latency in hidden platform test and swimming traces in all experiments were recorded for off-line analysis with software. After Morris water maze test, rats were anesthetized with urethane and positioned on a stereotaxic apparatus. A hole was drilled on the skull. A bond parallel electrode consisting of concentric stimulating electrode and recording electrode was inserted into the CA1 region of hippocampus to record field excitatory postsynaptic potentials(f EPSPs) in stratum radiatum by stimulating Schaffer collateral/associational commissural pathway. Basic synaptic transmission was first recorded for 30 min with test stimulation. High frequency stimulation was applied to induce L-LTP of f EPSP after basic recording. To test if presynaptic mechanism was involved, two successive stimuli with an interval of 50 ms were also applied to trigger paired pulse facilitation(PPF) before HFS.Results:Spontaneous alternation of Y maze was performed to examine the spatial working memory of rats. Aβ1-42 significantly decreased in the percentage of right alternation of rats(P<0.001) and leptin could effectively reverse this detrimental effect(P<0.001). At the same time, the total arm entries of rats did not show any significant difference between groups(P>0.05) suggesting the difference in spontaneous alternation between groups is involved in the change in spatial working memory, not the locomotor ability of rats.In consecutive 5 days of hidden platform tests, the escape latency in all groups showed a gradual decrease with the increase of training days. Significant main effects existed in Aβ1-42 vs. vehicle treatment(P<0.01) and leptin vs. saline treatment(P<0.01)on escape latency. There was also a significant interaction between Aβ1-42 and leptin groups. These results indicate that the rats in Aβ1-42 group spent longer time to find hidden platform than those in other groups, suggesting Aβ1-42 affected spatial learning ability of rats. A significant difference in memory retrieve showed between Aβ1-42 and control groups, Aβ1-42 significantly reduced swimming time of rats in target quadrant (P<0.001) and leptin reversed this effect(P<0.01). There is no significant difference in escape latency and swimming speed was found between these groups.The potentiation of f EPSPs was compared especially at 0 min, 60 min, and 180 min after HFS. Leptin had significant main effect on amplitude of f EPSPs at 0 min and 60 min.The results indicate that leptin facilitated the induction of in vivo L-LTP in our experiment.The L-LTP values in the last ten minutes were compared. There were significant main effects of Aβ and leptin, and significant interaction between leptin and Aβ1-42 groups on the mean amplitude of f EPSPs. Tukey’s post hoc test showed that the average L-LTP value was significantly suppressed by Aβ1-42(P<0.01) and this effect was reversed by leptin(P<0.001). We did not find any significant difference in PPF between groups(P>0.05).Conclusions:Injection of β-amyloid through lateral ventricle impaired spatial working memory and leptin could reverse this effect. Aβ1-42 could induce impairment of spatial reference memory and leptin reversed this effect. Leptin injection facilitated L-LTP induction and reversed Aβ1-42-induced L-LTP suppression via a postsynaptic mechanism. These results suggests that leptin may has a important role in treatment of Alzheimer’s disease.Alzheimer’s disease(AD) is a progressively neurodegenerative disorder afflicting more than 40 million people all around the world. The main pathological hallmarks of AD include amyloid plaques consisting of extracellular β-Amyloid(Aβ) and neurofibrillary tangles containing hyperphosphorylated tau in neurons. Although the exact pathogenesis of AD is still unknown, the increasing burden of Aβ in the cortex and hippocampus is closely correlated with memory loss and cognition deficits in AD. Dysfunction of spatial memory including navigation deficits and memory retrieval disruption appeared at the early stage of AD patients and rodent models. Memory dysfunction, including short-term and long-term spatial memory, is a prominent symptom of patients with AD. Direct injection of Aβ into the hippocampus or lateral ventricles in rats also led to a serious impairment in spatial cognition performance. Long term potentiation widely exists between neurons in central nervous system. It is thought to be closely related with learning and memory. Injection of Aβ protein into hippocampus or lateral ventricles of normal rodents also produced detrimental effects on hippocampal LTP induction. So, hippocampal LTP has been considered to be an important cellular mechanism of memory disruption.Leptin is a 16 k D hormone secreted mainly from white adipocyte tissue. The plasma concentration of leptin is positive correlated with fat mass, and the functions of leptin have been referred to many fields, such as food intake, energy homeostasis, bone formation,reproduction, immunity, insulin sensitivity and neural activity. In recent years, the neuroprotective effects of leptin in AD, Parkinson disease, epilepsy, ischemia greatly attracted the interests of neuroscientists.However, there is still lack of evidence whether leptin could protect the spatial memory and synaptic plasticity impaired by injectingβ-amyloid. Thus the present study investigated whether chronical intracerebral ventricular(i.c.v.) injections of leptin could reverse Aβ1-42-induced impairments in short term spatial working memory and long term spatial reference memory by testing the spontaneous alternation of rats in Y maze and monitoring the performance of rats in Morris water maze task. In addition, the in vivo hippocampal late-phase long-term potentiation(L-LTP), an important electrophysiological model of long term memory, was also examined following behavioral examination in rats.