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Impact of methodology on estrogens’ effects on cerebral ischemia in rats: an updated meta-analysis
Linköping University, Department of Clinical and Experimental Medicine, Division of Microbiology and Molecular Medicine. Linköping University, Faculty of Health Sciences. Örebro University Hospital, Sweden.
Linköping University, Department of Clinical and Experimental Medicine, Division of Microbiology and Molecular Medicine. Linköping University, Faculty of Health Sciences.ORCID iD: 0000-0001-8813-0384
2014 (English)In: BMC neuroscience (Online), ISSN 1471-2202, E-ISSN 1471-2202, Vol. 15, no 22Article in journal (Refereed) Published
Abstract [en]


Although most animal stroke studies have demonstrated potent neuroprotective effects of estrogens, there are a number of articles reporting the opposite. In 2009, we made the case that this dichotomy was related to administered estrogen dose. Several other suggestions for the discordant results have also been propagated, including the age of the experimental animals and the length of hypoestrogenicity prior to estrogen administration. These two suggestions have gained much popularity, probably because of their kinship with the window of opportunity hypothesis, which is commonly used to explain the analogous dichotomy among human studies. We were therefore encouraged to perform an updated meta-analysis, and to improve it by including all relevant variables in a large multiple regression model, where the impact of confounders could be controlled for.


The multiple regression model revealed an indisputable impact of estrogen administration mode on the effects of estrogens in ischemic stroke. Subcutaneous slow-release pellets differed from the injection and silastic capsule treatments in terms of impact of estrogens on ischemic stroke, showing that the first mentioned were more prone to render estrogens damaging. Neither the use of elderly animals nor the adoption of longer wash-out periods influenced estrogens' effects on experimental ischemic stroke in rats.


We conclude that the discordant results regarding estrogens' effects in rat models of ischemic stroke are a consequence of differences in estrogen administration modes. These results are not only of importance for the ongoing debate regarding menopausal hormone therapy, but also have an important bearing on experimental stroke methodology and the apparent translational roadblock for suggested stroke interventions.

Place, publisher, year, edition, pages
BioMed Central, 2014. Vol. 15, no 22
Keyword [en]
Cerebral ischemia; Estradiol; Estrogens; Meta-analysis; Rats; Stroke
National Category
Basic Medicine
URN: urn:nbn:se:liu:diva-106875DOI: 10.1186/1471-2202-15-22ISI: 000334885400001PubMedID: 24495535OAI: diva2:720080
Available from: 2014-05-28 Created: 2014-05-23 Last updated: 2016-02-29Bibliographically approved
In thesis
1. Challenges in experimental stroke research: The 17β-estradiol example
Open this publication in new window or tab >>Challenges in experimental stroke research: The 17β-estradiol example
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Ischemic stroke causes millions of deaths around the world each year, and surviving patients often suffer from long-term disability. Hundreds of promising drug candidates have been identified in animal models, but the clinical trials have repeatedly failed. Lack of methodological quality in the animal studies, e.g. low statistical power as a result of small group sizes in combination with high outcome variability and high mortality, has been suggested to in part explain the lack of translational success. In the meta-analytical Papers II and Paper V, we therefore investigated how method parameters impact infarct size variation and mortality in rodent stroke studies. These findings can help researchers to optimize their animal models or to more exactly predict variability and mortality given a certain experimental setup.

The relation between ischemic stroke and estrogens is complex. Premenopausal women have a lower risk of stroke than men of the same age, suggesting that female sex hormones provide protection against cerebrovascular events. The idea of a beneficial effect on the brain of estrogens was also supported by epidemiological studies showing that estrogens given as postmenopausal hormone replacement therapy decreased the risk of stroke. However, subsequent clinical trials reported the opposite, an increased risk. Interestingly, discrepancies exist also in the animal stroke literature. The majority of the rodent studies on the effects of estrogens have shown protection, but there are also several examples of increased damage. Based on experimental results and a meta-analysis, it was hypothesized that differences in hormone administration methods and their resulting plasma concentrations of estrogens might explain the previous discordant animal findings. Paper I investigated the commonly used methods for 17β-estradiol administration and found that the popular slow-release pellets produced high and unpredictable serum concentrations. A novel method with 17β-estradiol administered orally in Nutella® was also evaluated with promising results. Paper III extracted data regarding methodological choices from all previously published estrogen-stroke studies, and showed through metaanalysis that slow-release pellets are more prone to render estrogens damaging. Finally, Paper IV tested whether estrogens could both exert neuroprotection and promote detrimental effects merely depending on dose and irrespective of the administration route. Surprisingly, and in contrast to the hypothesis, a significant negative correlation was found between 17β-estradiol dose group and infarct size meaning that the higher the dose, the smaller the infarcts.

In summary, this thesis does not confirm the hypothesis of dose-related neuroprotective vs neurodamaging effects of estrogens on ischemic stroke. If high estrogen doses/plasma concentrations per se can cause increased stroke damage, such a phenomenon is not very robust, and seems to depend on tight dose ranges and/or other experimental circumstances. Although not directly applicable to the clinical situation, hopefully in a long-term perspective these findings may contribute in elucidating when estrogens are beneficial and when they are harmful. Further, it adds to the growing literature on how the quality of experimental stroke research can be increased to try to overcome translational difficulties.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 70 p.
Linköping University Medical Dissertations, ISSN 0345-0082 ; 1504
National Category
Basic Medicine Neurosciences
urn:nbn:se:liu:diva-123893 (URN)10.3384/diss.diva-123893 (DOI)978-91-7685-852-3 (print) (ISBN)
Public defence
2016-02-05, Berzeliussalen, Ingång 65, Campus US, Linköping, 09:00 (Swedish)
Available from: 2016-01-13 Created: 2016-01-13 Last updated: 2016-01-15Bibliographically approved

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