Accelerated Ovarian Failure: A novel, chemically induced animal model of menopause

Abstract

Current rodent models of menopause fail to adequately recapitulate the menopause transition. The intact aging model fails to achieve very low estrogen levels, and the ovariectomy model lacks a perimenopause phase. A new rodent model of Accelerated Ovarian Failure (AOF) successfully replicates human perimenopause and postmenopause, including estrous acyclicity and fluctuating, followed by undetectable, estrogen levels, and allows for the dissociation of the effects of hormone levels from the effects of aging. In this model, an ovotoxic chemical, 4-vinylcyclohexene diepoxide (VCD), selective for primary and primordial follicles, is injected intraperitonelly in animals for 15 days. As the mature follicle population is depleted through natural cycling, ovarian failure follows increasing periods of acyclity. Administered at low doses, VCD specifically causes apoptotic cell death of primordial follicles but does not affect other peripheral tissues, including the liver and spleen, nor does it cross the blood-brain barrier. In addition to reducing confounds associated with genetic and surgical manipulations, the AOF model maintains the presence of ovarian tissue which importantly parallels to the menopause transition in humans. The VCD injection procedure can be applied to studies using transgenic or knock-out mice strains, or in other disease-state models (e.g., ischemia, atherosclerosis, or diabetes). This AOF model of menopause will generate new insights into women's health particularly in determining the critical periods (i.e., a window of opportunity) during perimenopause for restoring ovarian hormones for the most efficacious effect on memory and mood disorders as well as other menopausal symptoms.

Modeling menopause: The utility of rodents in translational behavioral endocrinology research

“…Before we can fully appreciate rodents as valuable and appropriate models for menopause and aging research [48–49], we must first acknowledge some key differences between human and rodent reproductive senescence [23, 50]. Rodents have an estrous cycle rather than a menstrual cycle.…”

“…In the early 2000’s, VCD was introduced to biobehavioral animal research as a rodent model of transitional menopause because it was found to selectively target and deplete the non-growing ovarian follicle pool in rodents via atresia, resulting in accelerated follicular depletion and eventual ovarian failure in rodents [79–81, 84–93]. VCD is thought to function through accelerating atresia in primordial and primary ovarian cells — but not disrupting growing follicles — by altering the expression and distribution of the Bcl-2 family of proteins that regulate apoptosis, including Bcl-xL, Bax, and Bak proteins within ovarian follicles, thus initiating cytochrome c release into the cell cytosol, triggering downstream caspase signaling activity, all of which are related to accelerated follicle atresia [49, 84–85, 91–93]. …”

Sex differences in developmental programming models

“…Female rodents demonstrate cycle irregularities at around 9-12 months of age (Sokol et al 1999, Spencer et al 2008, eventually becoming acyclic and entering 'estropause' at 12-18 months of age (Van Kempen et al 2011). The key differences between menopause and estropause may be understood in terms of circulating oestrogen levels, which fall dramatically in human menopause, but are maintained at a higher ambient level in rodents (Maffucci & Gore 2006).…”

The Importance of Considering Sex Differences in Translational Stroke Research

“…Gonadectomy provides a tool for studying the effect of sex steroid hormones in stroke but do not recapitulate the overall aging phenotype as the loss of gonadal hormones is abrupt and complete, and the animals are still young, with young vasculature and brain. Chemical induction of menopause in female animals by 4-vinylcyclohexene diepoxide (VCD) better mimics the natural decline in female hormones and can be useful [132], but the use of aged animals is likely to be the most translationally relevant, despite their high cost. One should also be aware of the possibility of difference in body size between males and females, and between ovariectomized females treated with placebo versus estrogen, that can affect behavioral analysis and all tests should be performed in sham male and female animals and in experimental animals before initiating experiments if possible.…”