ABSTRACT Background: The effect of ovariectomy and the sudden loss of support for the spatial distribution of gonad hormones on the effectiveness of learning in young adult rats were examined. We hypothesized that spatial learning and memory in a considerable number of women who undergo surgical menopause and estrogen deprivation before their natural menopause is compromised. s Methods: In this study, we used 26 Wistar rats (approximately five months of age) and divided into two groups: intact and ovariectomized (OVX). They were tested for memory of spatial water maze Morris 6 weeks after OVX. Results: The results showed that the performance of OVX group in the water maze was significantly lower than the control group. Although the average has fallen across the path length in both groups of blocks, OVX rats were significantly longer than the length of the path through the control blocks 2-6 (P<0.05). OVX rats were less percent of total time spent in the quarter target controls in the probe test (P<0.05). Weight gain is significant only in OVX group during the experiment (P<0.05). Plasma estrogen significantly decreased after OVX (P<0.05). Conclusion: This finding provides further evidence of the role of estrogen, a gonad steroid hormone, in the handling of functions related to learning and memory. It is suggested that loss of estrogen after OVX faculties of spatial reference memory in young adult rats. Our results suggest that it is necessary to protect women who undergo surgical menopause before their natural menopause of cognitive disabilities. INTRODUCTION Menopause marks the beginning of a new phase in the life of a woman who is associated with a decrease in circulating estrogen levels. The average age at menopause has remained essentially constant at 50. Thus, 50-years, women spend nearly a third of their lives in an estrogen deficient state. This normal process of aging in women is associated with increased health problems such as osteoporosis, cardiovascular disease, and cancer and neurodegenerative diseases. Although estrogen deficiency has been linked to changes in several physiological processes, the extent to which the loss of estrogen is associated with cognitive changes noted by postmenopausal women has been more difficult to determine, for reasons business. In addition, the neuronal mechanisms that estrogen May affect cognitive function in women have not yet identified. As humans, rodents, exposure age-related cognitive decline, and thus provide good models for testing. It is the role of female sex hormones during May of cognition in the aging process. Over the past 25 years, the findings of basic neuroscience have provided us much information concerning the mechanisms of action of estrogen on brain structure and function. Estrogen has marked effects on hippocampal synaptic function. Estrogen increases hippocampal dendritic density and spine increases the number of varicosities that can make multiple synapses with different cells. In addition, estrogens may influence other signaling processes including the synaptic balance of protein kinases and phosphatases activity. Although estrogen increases performance on some tasks of learning, it is reached or no effect on others. The results of numerous studies indicate that estrogens exert a positive effect mainly on tasks that require the use of working memory, defined as memory for that information to a single test. For example, chronic estrogen replacement in ovariectomized (OVX) rats increased the number of visits to correct arm choices during acquisition of working memory tasks in the radial arm maze and increased the number of alternations in a reinforced T-maze.
Estrogen replacement in OVX rats had no effect on the choice of the reference arm of precision in the memory versions of a radial arm maze task. Estrogen replacement also improved the acquisition process and the position matchingto-delayed spatial memory tasks. On the other hand the positive effects of estrogen on working memory, many studies indicate that endogenous and exogenous estrogens impair or have no effect on the task depends primarily on reference memory, defined as memory of information consistent across the trials. For example, gonadally intact female rats and mice showed longer escape latencies to find a hidden platform that OVX controls in a reference memory Morris water maze. On the other hand, replacement therapy with estrogen reduces the number of errors in reference memory in radial arm maze years sham-operated and OVX mice, but unlike young mice, it had no effect on the errors of working memory. Memory induced by an early age reduces the OVX mouse estropausal closer to their age. Some evidence suggests that estrogen given to young OVX rodents can improve both spatial and non-spatial learning and memory. A study indicates that the cycle of estrogen replacement scheme does not affect spatial memory in young or middle-aged animals in the hippocampus-dependent appetitive radial maze task. Thus, the differences between these studies could be partly due to different ages of OVX and different effects on learning tasks and memory. In this study, we examined the effects of ovariectomy-induced estrogen reduction (or deprivation) on the reference memory Morris water maze in young adult rats. MATERIALS AND METHODS Issues: Female Wistar rats (n = 26), about five months, purchased from Ahvaz Jondishapour University of Medical Sciences (AJUMS) animals (Ahvaz, Iran). The mice were separately in temperature controlled vivarum 12 hours light / dark cycle (light on at 7.00 am). The animals were allowed free access to food and water. After one week, the animals were ovariectomized under anesthesia produced by injection of ketamine hydrochloride (90 mg / kg, ip, Rotex Medica, Trittau, Germany) and xylonite (10 mg / kg, ip, Miles Laboratories, Shawnee, Kansas, USA). All efforts made to minimize the number of animals used (Acost et al, 2009) Groups: Subjects were divided into two groups. The first (control) were gonadally intact, while the second was OVX. Morris water maze. The water maze was a black circular pool (140 cm in diameter, 70 cm in height) located in a well lit room and filled with water (50 cm height, 27 ? C). The maze performance was recorded by a video camera suspended above the maze and interfaced with a video (Teevanich Media Tracking System, Tehran, Iran). Extra maze cues surrounding the maze were fixed at specific locations and were visible to the rats. A platform (diameter 12 cm), was at the center of the northeast quadrant of the team that allowed rats to escape from the water. The escape platform was placed 2 cm below the surface (Daniel et al, 2006). Acquisition trials: Six weeks after surgery, OVX, water maze training began. In this task, the rats were trained to find a platform using extra maze cues submerged. Prior to water maze testing, all mice were habituated to swimming in water using three trial formulation process. This procedure habituated the rat to water and to teach that escape from the water by climbing to a platform.
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Subjects were trained in one day. Each rat received 18 trials over a period of 3 to 4 hours was for 20 minutes break between each 3 trials (6 blocks, each block consisting of 3 trials). The location of the submerged platform has not changed throughout the experiment. For each test, the subject entered the water facing the edge of the tank from random start points. In each test, the subject was 60 seconds to be allowed to escape from the submerged platform; Rats that failed to escape were led to the platform and the investigator were allowed to remain there for 15 seconds before removing it from the labyrinth and dried off (Davis et al, 2005). Probe trial: Following the acquisition of a day period, a probe trial was ordered. The probe trial was identical to acquisition trials with one exception. During the probe trial, the submerged platform was removed. Multiple measures of water maze performance were recorded. Swim distance (cm), quadrant time (percentage of time spent in each quadrant of the issue containing the platform) and swim speed (cm / s), 18 were recorded during testing and a probe trial (Harburger et al, 2005). Body weight and plasma estrogen: To confirm that the ovariectomy was effective, the record weight of the animal was kept at the beginning of the study and after six weeks and also measured plasma estrogens by ELIZA test 15 days after ovariectomy. Statistical analysis: Independent sample t-test performed to determine whether group differences existed in the proportion of time spent in the target quadrant and along the route during the acquisition and probe trials. The two-way ANOVA was run to determine if differences between groups for each block along the route was during the acquisition. A student of the paired t-test analysis was used to determine whether there were significant differences in the OVX group weight at initial and one month after ovariectomy. Student t-test performed to determine whether group differences in weight. All ex-post comparisons were calculated using the least significant difference method. The criterion for significance was P<0.05 in all statistical evaluations. DISCUSSION The results of this study indicate that ovariectomy in young adult rats affects the strategy used to find a hidden platform water discharge in the Morris maze. The average path length to find the leakage through the platform decreased in both groups of blocks, but path lengths OVX rats were significantly longer than controls. Percent of total time spent in target quarter increased across blocks in both groups but OVX rats had significantly lower percent of total time that controls the acquisition and probe trials. The Morris water maze task requires the involvement of multiple areas of neurons that are not involved in memory (Talboom et al, 2008) However, the absence of differences between groups in swimming speed during probe shows that the different performance tests between the groups is not due to differences between groups sensorimoteurs. Our results showed that both groups of rats are able to demonstrate learning through the blocks, but OVX rats learned task in a lower level of control intact rats. That finding by the loss of spatial memory in OVX rats was similar to previous reports. El-Bakri et al. reported that ovariectomy severely affect the spatial reference memory. Estrogen induced regulation of spatial memory and N-ethyl-D-aspartic acid (NMDA) are likely to be mediated by nuclear estrogen receptors, estrogen receptor %u03B1 (ER %u03B1) and %u03B2 (ER %u03B2). Both receptors are expressed in the hippocampus and the neocortex (Sarkaki et al, 2008) There is also the possibility of indirect effects through the estrogen interacts with other neurotransmitters such as the cholinergic system which in turn affect the glutamate system. It has already been shown that the ability of estrogen to alter the binding of CA1 NMDA receptors is linked to its ability to alter the cholinergic system. Previous studies show that estradiol plays a dual effect on NMDA receptors. It improves cognitive function, and at the same time exerts a neuro-protective effect. Thus, estrogen is responsible for changes in memory during the menstrual cycle. T. (Heikkinen et al, 2008). reported that ovariectomy affected the performance of aged mouse T-maze. In addition, Strus H. found that rats VX demonstrates superior performance on the maze task, as measured by latency to reach a goal (time) and error scores. Yamada et al. reported that neither long term (3 months) or short-term (1 month), estrogen deprivation by ovariectomy resulted in a significant impairment of spatial learning and memory in water, maze and spontaneous change of behavior in a Y-maze Wilson et al. suggested that short-term estrogen deprivation has no effect on spatial memory and reference, when it affects the spatial working memory. Many reasons can be advanced to explain these discrepancies in the results of research, such as differences in the type of memory that is being studied or the age of OVX animals. Based on some results, it is proposed that estrogen through an animal to use in the hippocampus or not, it is advantageous to do so. The hypothesis that estrogen influences cognitive May May selection strategy to provide a framework to explain why estrogens have positive effects on some tasks of learning and memory and alter or no effect on others. If a task is solved using a strategy based on estrogen May hippocampally improves performance (Pandaranandaka et al, 2008) However, if a task is best solved using a non-hippocampally strategy based on estrogen May affect performance. Finally, while some strategies are also effective in solving a task, the estrogen in May had no effect. In addition, the effect of estrogen on learning and memory depends on the age of the animal. It is suggested that many brain regions affected by estrogen, including the hippocampus, are sites of neurodegenerative disease age-related changes in both sexes, making May the elderly brain less sensitive to estrogen (Mitsushima et, 2008) Thus it seems that the long-term ovariectomy losing some of its effects as the female rats reached the age post-estropausal. The exact mechanism (s) by ovariectomy, which affects learning and memory are not clear. It is possible that the chronic loss of estrogen (and progesterone) May lead to a subtle decrease NMDA receptor binding and / or calcium signaling pathways in hippocampal CA1 dendrites. Carrer et al. reported that the slow after hyperpolarization (sIAHP) was significantly greater in cells from OVX rats than in cells of rats (Varadinova et all, 2008). In addition, they reported that the excitability of neurons from ovariectomis?s rats was significantly reduced compared to control rats and this effect was reversed by estrogen treatment. Oophorectomy can influence postsynaptic calcium ion signals in May, in turn, influence the balance between kinase and phosphatase pathways, and thus influence the dynamics of response to CA1 synaptic input.
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