Impact of estradiol on the central regulation of glucose homeostasis and subsequent implications for hippocampal function.

Project 4 Summary Midlife obesity and diabetes mellitus (DM) are risk factors for all dementias including Alzheimer’s disease (AD), vascular cognitive impairment, and vascular dementia. Loss of estrogens due to menopause results in decreased energy expenditure and impaired glucose homeostasis, leading to increased prevalence of Type 2 DM. Hypothalamic estrogen receptor signaling plays a role in the central regulation of energy homeostasis via modulation of both liver glucose production and tissue glucose uptake. Systemic glucose homeostasis is largely regulated by central autonomic circuits, and the risk of developing Type 2 DM is high if autonomic dysfunction is present. Autonomic dysfunction, including suppression of parasympathetic tone accompanied by sympathetic predominance and disruption of hypothalamic circuits involved in autonomic control of metabolism, has been observed in patients with several forms of cognitive impairment including AD. It is therefore crucial to determine cellular mechanisms that impair autonomic regulation of the liver and glucose homeostasis, and thus contribute to cognitive dysfunction. In addition, while obesity, insulin resistance and DM are known to impair hippocampal synaptic plasticity and cognitive function, there is a lack of comprehensive studies investigating the effects of midlife estradiol treatment on hypothalamic neurons involved in the central regulation of glucose homeostasis and on hippocampal function in the context of obesity. Our preliminary observations have revealed that estradiol treatment improves glucose levels in ovariectomized (OVX) animals. In addition, liver-related neurons in the paraventricular nucleus (PVN) of high-fat diet (HFD) treated mice are more active and HFD interferes with insulin- dependent suppression of excitatory neurotransmission in pre-sympathetic PVN neurons. Furthermore, estradiol enhances long-term potentiation (LTP) in hippocampal neurons of middle-aged OVX animals. These findings led to the central hypothesis that midlife estradiol treatment in OVX mice provides beneficial effects required to maintain proper function of the brain-liver pathway, hippocampal LTP, a cellular correlate for learning and memory, and hippocampus-dependent cognitive function, but these beneficial effects are attenuated in mice on HFD due to insulin resistance. The proposed studies will determine the effect of midlife estradiol treatment on 1) the cellular properties of preautonomic PVN neurons, 2) synaptic plasticity and cellular properties in the hippocampus, and 3) glucose homeostasis and cognition in control and HFD mice. These studies may lead to new strategies to improve glucose homeostasis during cognitive impairment as well as to a better understanding of the effect of midlife estradiol treatment on hypothalamic and hippocampal function.