Effects Of Estrogen Replacement Therapy On Cognitive Function And Myelin Sheaths In Hippocampus Of Middle-aged Ovariectomized Rats

Numerous studies have provided evidence that estrogens exert directeffects on the central nervous system，such as on learning, memory andmood. Epidemiologic studies have showed that post-menopausal womenhad a higher risk of Alzheimer’s disease(AD) than men, while estrogenreplacement therapy initiated in menopausal could delay progress ofdementia and improve cognitive performance. In our study, middle-agedfemale rats were bilaterally ovariectomized. One month after ovariectomy,successfully ovariectomized rats were randomly divided into placebo group(OVX+O) which received sesame oil injection, and estrogen replacementgroup (OVX+E) which received continuous17β-estradiol replacementtreatment. Spatial learning capacities of two group rats were assessed usingthe Morris water maze. The myelinated nerve fibers and myelin sheaths inCA1region were quantitatively investigated with transmission electronmicroscope technique and stereological methods. Part One1.Materials and methods1.119female Sprague-Dawley rats（10～12months old）underwent OVXsurgery. Vaginal smears and serum estradiol levels were used to determinewhether the ovaries were fully removed. The OVX rats were randomlydivided into two groups. One was OVX+E group (n=10), and the otherwas OVX+O group(n=9).1.2Rats in OVX+E group received continuous17β-estradiol replacementtreatment for4weeks. Rats in OVX+O group received sesame oil injectionfor4weeks. Serum estradiol levels were detected at the last injection.Uterine tissues were collected and weighed after sacrificed.1.3The spatial memory of animals was tested with Morris water maze task.2. Results2.1Significantly lower serum estradiol levels and uterine atrophy wereobserved in OVX+O rats, when compared with those in OVX+E rats(P<0.05).2.2The escape latency in Morris water maze task was significantly shorterin OVX+E group when compared to OVX+O group (P<0.05).Part Two1.Materials and methods1.1Six rats were randomly selected from OVX+E group, and five rats wererandomly selected from OVX+O group.1-mm-thick consecutive slabswere sliced from cerebral hemisphere and photographed under anatomical microscope.1.25～7tissue blocks randomly sampled from CA1region were cut intoultra-thin sections. From each section,5-10fields of view were randomlyphotographed under transmission electron microscope with a magnificationof20000. There were50photographs from each animal.1.3The volume of CA1region, the volume of the myelinated nerve fibersand the volume of the myelin sheaths in CA1region were quantitativelyinvestigated with the stereological methods.1.4The diameter of myelinated nerve fibers, the diameter of axons and themean thickness of the myelin sheaths in CA1region were quantitativelyinvestigated with the stereological methods.2. Results2.1The volume of CA1region was26.83±1.17mm3in OVX+O group and27.85±1.15mm3in OVX+E group, with no significant difference betweentwo groups (P>0.05). There were no significant differences in the volume ofthe myelinated nerve fibers (P>0.05) and the volume of the myelin sheaths(P>0.05) in CA1region between two groups.2.2There were no significant differences in the diameter of the myelinatednerve fibers (P>0.05) and the diameter of the axons (P>0.05) in CA1regionbetween two groups.2.3The mean thickness of the myelin sheaths of the myelinated fibers withdiameters between0.6to0.7μm was significantly higher by14.1%in OVX+E group when compared to OVX+O group (P<0.05). The differencebetween the diameter of the myelinated nerve fibers and the diameter ofthe axons with diameters between0.8to0.9μm was significantly higher by18.2%in OVX+E group when compared to OVX+O group (P<0.05).General conclusions1. Estrogen replacement therapy could improve spatial referencememory in middle-aged female rats.2. There was no significant difference in the volume of CA1regionbetween OVX+O rats and OVX+E rats.3. This study provided the basic data of the volume of the myelinatednerve fibers, the volume of the myelin sheaths and the mean thickness of themyelin sheaths in CA1region of middle-aged OVX+O and OVX+E rats.These data could provide the scientific basis for the future studies on theeffects of estrogen replacement therapy on aged brain and on the brain inneurodegenerative diseases.4. Estrogen replacement therapy could prevent the myelin sheaths lossand/or promote myelination of the myelinated fibers with middle rangediameter in CA1region.