The Effects Of Long-term Hormone Therapy Initiated At Different Stage Of Menopause On Cognitive Function In Ovariectomized Middle-aged Rats

Background:Findings from basic neuroscience have provided us with a great deal of information concerning the mechanisms of action of estrogen on brain structure and function. Estrogen receptors(ER) are widespread. And they are found in cerebral cortex, hippocampus, midbrain and other areas. Estrogen may directly modulate brain development and aging, affect neurochemical systems and synapse functions involving in cognitive decline at menopause.Hormone replacement therapy (HRT) have been used for postmenopausal women for over 50 years. The argument about the benefits and risks of HRT is never stopping. Observational studies suggested that HRT might protect postmenopausal women against cognitive decline and AD. However, the results of randomized controlled trials in women age 65 years and older were negative. Several clinically important questions remain unanswered, including questions on the efficacy of estrogen on cognition to groups of women for whom HT is an indication-perimenopausal women and those soon after menopause who have menopausal symptoms.Perhaps, there is a critical window for medication. The protective effect of estrogen on neurons involved in memory processes presents only when estrogen is given soon after the menopause, but not long after cessation of ovarian function. This study was designed for investigating the effects of long-term hormone therapy initiated at different period of menopause on cognitive function by ovariectomized (OVX) middle-aged rats.Objective:To investigate the HRT efficacy initiated at different stage of menopause, it was assessed on behavioral changes, the function of neurotransmitters and their receptors, morphologic and ultrastructure changes of neurons and synapses in OVX rats.Methods:Female Sprague-Dawley rats were ovariectomized at 9-10 months of age. They received CEE plus MPA replacement beginning either immediately (early stage of postmenopause) or 4 months (late stage of postmenopause) after ovariectomy. The dosages are CEE0.07mg/kg/d or 0.5mg/kg/d, MPA 0.2mg/kg/d. Morris water tests were used to assess the spatial memory function and neurochemical tests were used to detect related enzyme and receptor, immunohistochemistry methods were used to evaluate the expression of nNOS and Aβand electron microscope were used to observed the ultrastructure of neurons and synapses.Results:1. The estrus circle disappeared, the weight increased significantly in water OVX group rats. HRT can easily reverse the gain of weight in early stage, not in late stage.2. The effect of age and timing in relation to menopauseExcept for the memory scores, all the behavioral data in early controls were similar to the late controls(P＞0.05 ). The ChAT and MAO activity, nNOS and Aβexpression, counts of neurons in cerebral cortex and hippocampus were better in early stage than those in late stage (P＜0.05) .The NMDA-R content in hippocampus between two groups was similar. (P＞0.05)3. The HRT efficancy in different stage of menopauseIn early stage, the latency, distance, memory scores and correct strategies rates were better in HRT-treated rats than in water-treated rats (P＜O.05). The NMDA-R contents,nNOS positive area density and synapses counts in HRT group were increased, and Aβpositive area density was decreased as compared with water group in cerebral cortex and hippocampus (P＜O.05). MAO activity were inhibited, ChAT activity and neuron counts were enhanced at hippocampus in HRT groups compared with water group (P＜O.05).The comparison between the two dosage HRT groups had no difference (P＞0.05) ,while ChAT activity and neuron counts in frontal cortex were significantly increased in Iow-HRT instead of high-HRT groups as compared with water-groups.In late stage, only NMDA-R level and synapses counts could be increased significantly in high-HRT groups, others data in HRT groups were similar to those in water groups. The comparison between the two dosage HRT groups had no difference (P＞0.05) . HRT had no effect on MAO activity (P＞0.05)In area of frontal cortex and hippocampus CAl, capillary and neuron ultrastructure were abnormal in early water group and late stage groups, however, these changes were much milder in early HRT groups. 4. Interaction between the initiated time of HRT and the dosages of HRTExcept for the speed,M-R content and MAO activity in cortex, other data were effected by the initiated time of HRT or the interaction between the two factors. All the data in early stage were better than late stage(P＜0.05 ) ,and HRT efficacy in early low-HRT group was more or equal to high-HRT group.Conclusion:Our studies indicated that HRT initiated in early stage can protect the spatial memory, increase ChAT activity; inhibit the overexpression of MAO activity in hippocampus, enhance the special binding of NMDA-R ,maintain the neuron and synapses counts ,regulate the NMDA-NO signal transduction pathway and the expression of Aβand thus increase the cognitive and memory. HRT has no effect on M-R and MAO activity in cerebral cortex. At 4 months after OVX, except for enhancing the special binding of NMDA-R with high-HRT treatment, the major neuroprotective effects were weak or disappear.Different neurons and transmitters have different responses to the estrogen deprivation. Before HRT treatment, the onset of early and late stage is different, so the efficacy of HRT is different.We also indicated that the efficacy of low-HRT initiated immediately after OVX maybe have the best choice on neuroprotective function.