Reproductive age modulates the impact of focal ischemia on the forebrain as well as the effects of estrogen treatment in female rats

Abstract

While human observational studies and animal studies report a neuroprotective role for estrogen therapy in stroke, the multicenter placebo-controlled Women's Health Initiative (WHI) study concluded that hormone therapy increased the risk for stroke in postmenopausal women. The present study therefore tested the hypothesis that estrogen replacement would increase the severity of a stroke-like injury in females when this replacement occurs after a prolonged hypoestrogenic period, such as the menopause or reproductive senescence, but not when given to females that were normally cycling immediately prior to the hormone replacement. Two groups of female rats were used: multiparous females with normal but lengthened estrus cycles (mature adults), and older multiparous females currently in a persistent acyclic state (reproductive senescent). Animals were either used intact, or were bilaterally ovariectomized and immediately replaced with a 17β-estradiol pellet or control pellet. Animals were subject to a forelimb placing test (a test for sensorimotor deficit) and thereafter to middle cerebral artery occlusion (MCAo) by stereotaxic injection of the vasoconstrictive peptide endothelin-1, adjacent to the MCA. One week after stroke, behavioral tests were performed again. Cortical and striatal infarct volume, measured from brain slices, was significantly greater in intact reproductive senescent females as compared to intact mature adults. Furthermore, estrogen treatment to ovariectomized mature adult females significantly reduced the cortical infarct volume. Paradoxically, estrogen treatment to ovariectomized reproductive senescent females significantly increased cortical and striatal infarct volumes as compared to control pellet replaced senescent females. Significant post-stroke behavioral deficit was observed in all groups on the side contralateral to the lesion, while senescent females also exhibited deficits on the ipsilateral side, in the cross-midline forelimb placement test. Using an animal model that approximates the natural ovarian aging process, these findings strongly support the hypothesis that the effectiveness of estrogen therapy in protecting brain health may depend critically on the time of initiation with respect to a female's reproductive status.

Interrelation between inflammation, thrombosis, and neuroprotection in cerebral ischemia

“…This advantage is abolished in ovariectomized animals, due to the loss of endogenous female sex hormones (Simpkins et al , 1997 ;Alkayed et al , 1998Alkayed et al , , 2000Hawk et al , 1998 ;Liao et al , 2001 ;Gibson et al , 2005 ;Park et al , 2006 ;Dra č a, 2009 ;Selvamani and Sohrabji , 2010 ), and the consequences of cerebral ischemia in aged animals are more severe than in young animals (Davis et al , 1995 ;Alkayed et al , 1998 ).…”

“…17 β -Estradiol salvages the brain from ischemic injury, even enhancing recovery and reducing infarct size in ovariectomized (Simpkins et al , 1997 ;Selvamani and Sohrabji , 2010 ) and reproductively senescent or aged females (Alkayed et al , 2000 ;Liao et al , 2001 ), as well as in male animals (Hawk et al , 1998 ). In the case of acyclic females, this is subject to administration of 17 β -estradiol at the onset of senescence or ovariectomy and not in older acyclic females (Bake and Sohrabji , 2004 ;Suzuki et al , 2009 ;Selvamani and Sohrabji , 2010 ).…”

“…In the case of acyclic females, this is subject to administration of 17 β -estradiol at the onset of senescence or ovariectomy and not in older acyclic females (Bake and Sohrabji , 2004 ;Suzuki et al , 2009 ;Selvamani and Sohrabji , 2010 ). This seems to be due to the upregulation of estrogen receptor alpha (ER α ) close to the onset of senescence, in response to declining estrogenic stimuli from the ovaries, inadvertently providing a substrate for exogenous estrogen.…”

“…This seems to be due to the upregulation of estrogen receptor alpha (ER α ) close to the onset of senescence, in response to declining estrogenic stimuli from the ovaries, inadvertently providing a substrate for exogenous estrogen. However, in older acyclic females, the ER α is already downregulated, and thus, exogenous estrogen becomes deleterious (Jezierski and Sohrabji , 2001 ;Selvamani and Sohrabji , 2010 ). Ischemic injury itself has been found to increase the expression of ER α in the cerebral cortex, without infl uencing ER β expression.…”

“…Sex hormones not only alter procoagulant protein expression (Lowe et al , 2004 ) and the function of blood and vascular cells (Kadir et al , 1999 ;Butenas and Mann , 2002 ;Leng et al , 2004 ), but differences in platelet function (Liao et al , 2001 ;Suzuki et al , 2009 ) and in thrombosis activity (Bailey et al , 2009 ) have also been noted. Estrogen is to a degree neuroprotective (Liao et al , 2001 ;Suzuki et al , 2009 ;Selvamani and Sohrabji , 2010 ) in certain cases of induced cerebral ischemia, as females appear to suffer less severe consequences of stroke, including lesser neural tissue loss, than their male counterparts (McCullough and Hurn , 2003 ;Suzuki et al , 2009 ). In the absence of ovarian hormone production at menopause, females are again at higher risk to strokes than their male counterparts, and this risk continues to increase with age, as women have a longer life expectancy than men (Mitka , 2006 ;Suzuki et al , 2009 ).…”