Alterations in the hypothalamic-pituitary-ovarian axis in perimenopausal women are associated with multi- organ risk factors for disease, yet the biological mechanisms underlying this increased disease risk are largely unknown. This proposal addresses unanswered questions regarding the vulnerability of the middle- aged brain to global ischemia. In young female rats, the presence of physiological levels of estradiol before and after global ischemia, as might occur during cardiac arrest, reduces hippocampal CA1 neuron loss and associated cognitive impairments. Whether estradiol retains its neuroprotective actions in middle-aged females, and whether the age-related decline in insulin-like growth factor-l (IGF-I) increases vulnerability to ischemia-induced neurodegeneration and cognitive impairment, are unknown. This proposal aims examines the roles of age, estrogen and IGF-I in the survival and function of hippocampal neurons in a rat model of global ischemia. The underlying hypotheses are (1) that the middle-aged brain retains its responsiveness to the neuroprotective actions of estradiol if the duration of estrogen withdrawal is brief (""""""""critical period hypothesis"""""""") or circulating levels of IGF-I are maintained, and (2) that estrogen acts in the middle-aged brain to activate specific cell survival pathways and thereby intervenes in apoptotic cascades to prevent death of neurons otherwise """"""""destined to die"""""""".
Specific Aim 1 uses stereological cell counting and behavioral tests to evaluate the outcome of global ischemia in middle-aged female rats that are intact, ovariectomized at various intervals prior to insult, or ovariectomized and treated with estradiol at various intervals after ovariectomy. If estradiol does not preserve neurons and cognitive function in older hormone-deprived animals, we, will also determine if IGF-I can reinstate estrogen protection.
Specific Aim 2 examines the apoptotic death cascades triggered by global ischemia and identifies the site at which estrogen intervenes in these cascades. We will examine 1) mitogen-activated protein kinase and cAMP response element binding protein at early times after ischemia;2) the anti-apoptotic gene Bcl-2 and activation of caspase 3 at later times after ischemia;3) inactivation of Akt and subsequent activation of the forkhead transcription factor FKHRL1 at early times after ischemia. These experiments will provide new information on the potential for hormone therapy instituted during the perimenopausal transition to protect the brain from damage due to global ischemia.
